(covers information from several alternate timelines)
A deflector field was the energy field created by the deflector array of a starship. A deflector field was typically initialized automatically, though in extreme circumstances, it had to be activated manually.
When the USS Defiant's computer systems were rendered inoperable by a cascade virus in 2373, the deflector field had to be manually initialized by the bridge crew when the ship undocked from Deep Space 9. (DS9: "For the Uniform")
The deflector field had to be manually activated on a severely damaged USS Voyager during the Year of Hell, after a Krenim scientist altered the past in 2374. This timeline was negated when the Krenim weapon ship was deleted from history. (VOY: "Year of Hell, Part II")