I was about to post that it sounds like a shear pin or bolt. (Not "sheet", the parts man won't know what you need.)
Pop the top off of the leg. If the crown gear turns when you hit the switch, but the leg doesn't move, dig around in the grease and find a pin (or broken parts of a pin) about 1/8" diameter and an inch long.
There is a hole through the gear and a matching hole in the top of the lead screw. The pin locks the two together.
The pin is usually broken when the landing gear is run too far up or down. The system can, however, keep working for some time until all of the parts work their way out of the holes and everything fails.
Good luck.