I've had a couple Widgets with stuck heads that would travel a little bit, but not through the entire range. I've seen two different causes of this problem.
First, the brake might be miscalibrated where it releases the heads enough to move a little bit, but not enough to allow them to move through the entire range. So recalibrating the brake would help in a situation like this.
Second, there might be a spot of resistance in the heads' travel that the voice coil isn't strong enough to move the armature through. Try moving the heads manually (with the drive spun up!!!) to see if there's any resistance near either end of its travel. If there is, move the heads back and forth through the tough spot(s) a few times and things will hopefully loosen up enough to bring the drive back to life next time you turn it on. This shouldn't damage anything as long as the disk is spinning properly. To be extra safe, maybe unplug the 8 pin DIP socket with the voice coil ribbon cable coming out of it before you try to move the heads with your finger!