We've got a deadbolt that decided to stop working today. When we turn with either the key (outside) or the lever (inside), the knob/key moves the full motion, but the deadbolt only moves about 1/2 an inch. It does not retract all the way inside the door, and therefore we can't get the door open.

As I said, the movement seems fluid, but the deadbolt just doesn't move far enough. What would cause this, and how might I go about fixing it?