One out of 2:

1) If you are lucky, its only the smal gear that the wire connects to thats broken, its a easy fix, up with the old, down with a new
http://img.photobucket.com/albums/09...Girkasse_r.jpg

2) The plastic gear inside your gearbox has failed (mine did).
Replacing means dropping the gearbox, and taking it apart! (or get a replacement gearbox)
Yue can see the brown plastic gear here, next to the dif bearing:
http://img.photobucket.com/albums/09...edo-gear_r.jpg

Have a look at some pics from my rebuild here:
http://photobucket.com/albums/0903/Tombra/Girkasse/

Lets hope its just the replacement gear thats failed!
Good luck

Edit: They are both plastic