(covers information from several alternate timelines)
In 2371, the USS Voyager discovered a planet in the Delta Quadrant whose inhabitants had all been killed by a polaric ion explosion. The shock wave from the explosion, unlike most shock waves which dissipate into the future, dissipated into the past, causing subspace fractures to form across the surface of the planet. Investigating, Captain Janeway and Lieutenant Paris became caught in one of these fractures, and were transported back in time approximately one day before the explosion. In an attempt to rescue their crewmates, the Voyager crew constructed a device that could successfully widen a subspace fracture for Janeway and Paris to return through.
Returning to the planet, they initiated the device and opened a fracture, which Janeway observed in the past. Noticing that, as the fracture grew larger, it was in danger of intersecting with a conduit wall in a polaric ion power plant, Janeway realized that it was the Voyager crew's own rescue attempt which caused the explosion in the first place. Using her hand phaser, Janeway managed to seal the fracture, thereby preventing the explosion and changing history in that Janeway and Paris were never transported back in time in the first place. (VOY: "Time and Again")