1981 Ford Fairmont 6cyl auto transmission.
It is a C4 auto transmission.

I started my vehicle, put it into reverse
gear, there was a noise like a snap and that was it.
Looked under the vehicle to see if drive shaft was turning
when in gear, thinking it might be the differential, however
the drive shaft does not rotate.

I am assuming it is the transmission or could it be something
else?
I am not low on fluid check it regularly.
This vehicle has been maintained over the years however
things go wrong, and this is the situation right now.

Thanks in advance
Denny