[Adrian Clark]

If you stripped a gearset you would be having the exact opposite of the problem you described. If it's locked up it's probably because a set screw or keyway has come loose inside the gearbox, as mentioned before.

Have you disassembled the gearbox yet? You'll get much more information from the gearbox itself than you will from a bunch of CD'ers trying diagnose your problem based off of one symptom.

Edit: In hindsight it was probably the external self-locking retaining ring popping off the CIM shaft and getting jammed between the gears. But if there was a grinding sound beforehand then you might have larger problems.