Have you opened it up and see if you stripped the gears on the servo? Chances are the teeth is stripped and its binding the rest of the gears in the servo hence the servo is stick in one position.

You can buy replacement gears instead of a new servo, or, if your lucky, do what I did and rotate the broken gear so that the full travel of the servo JUST misses the stripped teeth.
https://www.youtube.com/watch?v=pkGYHjp4G5s