Make sure the transmission is not in "bypass" mode, meaning the little lever on the back should be pushed in not pulled out. If that's in the correct position, pull the lever out, make sure the parking brake is not set, then see if you can roll the tractor. If you can't there is a chance the brakes are locked up. It's worth mentioning that if either of the rear wheels have been removed from the tractor recently it's very possible that one of the wheel keys didn't get put back or got pushed out when the wheels were put back on.
If all those things look good, I'd recommend having a technician come look at the tractor because the transmission could have failed.