You haven't had a wheel off recently, have you? I've seen people remove a wheel to work on the tire and lose the key.
That unit is about the age to have the input pulley with the 6 sided or 8 sided hub. I have seen a hub strip out and the pulley then just turned around the input shaft.
You said the belt was still turning the input pulley.
If neither of these is the problem, the problem is internal. I don't know if there are any repair parts available for those transmissions or not.