Stasis lock is a critical state of being for Transformers. Stasis lock is a sort of coma that allows a Transformer to stay alive, running at the lowest level of power, despite massive energy loss or traumatic damage. Stasis lock can come about in a few different ways.
The first way is from a shock of massive damage. When the Ark crashed on Earth all of the occupants on board were retconned into emergency stasis lock. At various times during the Beast Wars combatants had to be returned to their respective bases due to going into stasis lock after being badly damaged.
The second way seems to be a more gradual process and can come about from damage and energy depletion over a longer period of time. Dinobot was told by his internal computer to go into stasis lock or risk losing his spark. This method is a conscious decision, and the Transformer can choose to override stasis lock, unlike the first way which seems to be an unconscious action by the Transformer's on-board computer.
In Transformers Animated, "stasis lock" is used in a different sense — through the use of stasis cuffs, a Transformer can be prevented from moving or using any special powers they possess — without being rendered unconscious or incapable of speech. Sentinel Prime, when placed in this condition by Optimus Prime, referred to this as "stasis lock".
Revenge of the Fallen
Jetfire, a very ancient ex-Decepticon Seeker entered stasis lock at some point on Earth while in the form of the SR-71 Blackbird and ended up in the Air and Space Museum. It is unknown exactly why he entered it but it is probably because of his extreme old age. When Sam, Mikaela, Leo and Simmons found him, an Allspark shard revived him and brought him out of stasis lock. Coming out of stasis lock Jetfire displayed no injuries that could have sent him into it (although the Allspark shard could have healed that), but had a very hard time getting around and malfunctioned. Despite this, Jetfire proved to be a very useful source of information and even in his weakened state, proved to be an effective fighter killing both Mixmaster and Scorponok although his cannons apparently didn't work (and one was shown malfunctioning after he was revived) as he used his fists and battle-axe.
In the Revenge of the Fallen novel, Optimus wasn't killed like in the film, he was simply sent into stasis lock for a very long time by even Transformer standards. In that version, he was only stabbed through the back and not blasted also. When Sam brought him back in the book version, all he did was bring Optimus out of stasis lock, not resurrect the dead like in the movie version.