(covers information from several alternate timelines)
In numerous alternate timelines, "The Year of Hell" was the name that was given to the events taking place in 2374 by the crew of the USS Voyager, specifically a series of conflicts with the Krenim Imperium. In both timelines, the temporal weaponry of the Imperium inflicted critical damage on the starship.
In this timeline, first contact with the Krenim took place on stardate 50973, when a single vessel launched an attack on the starship. Voyager's weapons fire was ineffective against the attacking vessel, and chroniton torpedoes were able to penetrate the hull. Hull breaches were recorded on Deck 7, and the engineering station on the bridge exploded, killing Captain Kathryn Janeway and Chief Engineer B'Elanna Torres. Nine other casualties were recorded, with power down on Decks 4-12, The Doctor going off-line as a result (he remained out of action for the remainder of the year). The mess hall was turned into a makeshift medical facility. Neelix was forced to wear an operations division uniform, eventually becoming a full-fledged security officer.
One torpedo failed to detonate during the battle, and became lodged in Voyager's hull on Deck 11, section 2. Fragments of the torpedo leaked chroniton radiation into the ship, irradiating a large portion of the crew. As a result, the entire crew was inoculated, but residual chronitons remained dormant in those affected.
Following the initial battle, a protracted conflict took place, inflicting severe damage on Voyager. Chakotay took command of the ship, ultimately managing to escape Krenim space. During the year, Kes and Tom Paris became romantically involved, producing a daughter named Linnis, who later gave birth to Andrew Kim. At the time of her birth, the main computer core had been off-line for several weeks, life support was restricted to three out of fourteen decks, and non-essential power had been rerouted to tactical systems. When The Doctor was brought back on-line, he was given hair and choose the name "Dr. Van Gogh".
The timeline was apparently averted when Kes' chroniton contamination was triggered by The Doctor's attempts to save her life with a bio-temporal chamber in 2379, resulting in her traveling backwards along her personal timeline to a point in early 2373, prior to the encounter with the Krenim. She later documented her knowledge of the Krenim in a report. (VOY: "Before and After")
In this timeline, first contact with the Krenim was significantly delayed, as Voyager first encountered the Nyrians and the Borg. (VOY: "Displaced", "Scorpion") These events resulted in the departure of Kes and the placing of Seven of Nine aboard the starship. (VOY: "The Gift") This timeline continued to evolve very differently, culminating in the destruction of Voyager, and the eventual erasure of this timeline. (VOY: "Year of Hell", "Year of Hell, Part II")
Two hundred years prior, Annorax, a Krenim scientist, had developed a temporal weapon ship capable of altering history. Using it to eliminate their greatest rivals, the Rilnar, Annorax failed to fully understand the complexities of the alteration, and a deadly virus devastated the Imperium. Trying again, Annorax was able to restore 98% of the Imperium, with the exception of Kyana Prime, home of his wife, who was erased from existence as a result. Annorax and his crew spent the next two centuries attempting to correct that mistake. In March of 2374, the Krenim's sector of space was dominated by the Zahl, who, in that timeline, had defeated the Imperium a number of years earlier.
In December of 2373, when attempting to evade a rogue comet, Voyager altered course to a heading that took the ship into Krenim space. In this timeline, first contact with the Krenim took place on 16 March (Day 1), when a single vessel launched an attack against the starship. Due to temporal interference from Annorax, the Krenim Imperium was non-existent and there was no significant damage. A few days later, during Voyager's meeting with a Zahl representative, Annorax altered time again to restore the Imperium and erase the Zahl. Voyager was significantly more damaged and on red alert, with shields failing. The second Year of Hell began.
On 17 April (Day 32), a Krenim attack resulted in the overload of EPS conduits across the starboard side of Deck 5, sections 10-53, killing two crew members. Voyager happened to take down the aft shields of the Krenim warship during the battle and destroyed it by releasing photon torpedoes like mines, but apparently another ship took its place. Attempts to modify Voyager's shields to protect against the temporal flux of the chroniton torpedoes had proven unsuccessful, phaser banks were burned out, and torpedo launchers were off-line. 2 May (Day 47) saw nineteen main power relays severed, shutting down the turbolift network. Later, Seven of Nine discovered one chroniton torpedo that had failed to detonate in battle, and was able to carry out a more direct analysis of it to work on providing Voyager with a suitable defense. The torpedo later detonated, blinding Lt. Commander Tuvok permanently as he attempted to get Seven to safety. At the same time, Tom Paris came up with an idea to use transverse bulkheads in order to minimize damage.
By 20 May (Day 65), the power grid on Deck 11 had been completely destroyed, rendering the replicator system nearly inoperative, limited to emergency rations only. Seven decks had lost life support, forcing the surviving crew into close quarters. It was at this point that the temporal shielding came online, limiting further damage. At this time, Neelix was forced to wear an operations division uniform, eventually becoming a full-fledged security officer.
As Annorax attempted another temporal incursion at the Garenor homeworld, Voyager's new shields interfered with the event, reducing the Imperium to a pre-warp civilization. This brought the Federation starship in direct contact with the weapon ship, as Annorax sought to eliminate Voyager from history. Although Annorax captured Chakotay and Paris as 'samples' from Voyager, the ship's temporal shielding protected it long enough to engage warp 7 and escape- the temporal weapon ship only capable of warp 6 at best- but the resulting warp stresses caused serious damage to the outer hull before Voyager returned to impulse.
With life support failing and the ship no longer able to sustain its crew, Janeway ordered the evacuation of Voyager on 28 May (Day 73); the senior staff remained on board, and subsequently took refuge in a class-9 nebula. By 27 July (Day 133), only one warp nacelle was functional, and the ventilation system was failing, allowing nebular gas to penetrate the ship. The remaining crew (Janeway, Kim, Seven, Torres, Tuvok and Neelix) drank "the elixir of endurance" and kept on going.
On 12 September (Day 180), a micrometeoroid storm in open space almost caused the loss of one of the nacelle pylons, save for the last-second reinitializing of the navigational deflector, which brought the ship's inoperable shields online after Captain Janeway's emergency manual repairs fixed the deflector and the shields. Reactivating the deflector left Captain Janeway with severe burns.
Seeking allies to eliminate the weapon ship, Voyager was able to forge an alliance with the Nihydron and the Mawasi; a task force, led by Voyager, was dispatched to track down and destroy the vessel, finally locating it on 29 October (Day 226), thanks to a coded message from Paris. Voyager's crew was divided up among the taskforce, with Janeway remaining on Voyager to coordinate the attack. Engaging the ship on 29 November (Day 257), Voyager suffered the loss of several sections of the dorsal saucer, including the forward bulkhead of the bridge, when Annorax opened fire with conventional weapons, causing a Nihydron vessel to collide with the Starfleet ship. The weapon ship's temporal core was deactivated by Obrist, Annorax's first officer, leaving it vulnerable. But with the other vessels disabled or destroyed, and torpedo launchers off-line and Voyager itself mortally wounded from a massive hit, Janeway had no choice but to set a collision course with the weapon ship in order to destroy it. Voyager impacted over the temporal core (of which Paris had sent the location) and exploded. Voyager was destroyed and the temporal core started to destabilize.
The impact, and the resultant destabilization of the temporal core, caused a temporal incursion within the weapon ship, erasing itself and all of its previous changes. As a result, Voyager was warned away from Krenim space on Day 1 by the same ship they originally encountered (with much less belligerency), and were able to plot a course around the Imperium. (VOY: "Year of Hell", "Year of Hell, Part II")
In the Voyager relaunch novel A Pocket Full of Lies, the crew learn about the Year of Hell in the true timeline when they encounter an alternate version of Janeway created during the events of "Shattered" as the result of a Krenim attack; the Krenim manipulated Janeway into believing that she had been abducted by the Zahl and rescued by the Rilnar to keep her confined. In the course of their investigation, the crew deduce that, in the 'original' timeline, Voyager used Seven of Nine's expertise to plant various transphasic log buoys throughout Krenim space that were protected from the changes in the timeline, each encoded to only respond to Janeway's access codes if recovered in this specific quantum reality, with the Krenim recovering some of these probes and learning about Janeway's role in Annorax's defeat while the USS Vesta manages to recover one of the last probes while investigating the alternate-Janeway's existence. These probes include various log records of events such as Janeway ordering the rest of the crew to leave and the Doctor attempting to order Janeway to stand down due to being unfit to command, but naturally do not include the final outcome of this timeline, although the fact that they exist in this manner allows Janeway to confirm that she defeated the temporal weapon ship in some way.