If you can get a look at the hydraulic fluid, you'll know whether it has been overheated. Of course it is possible that it was both overheated and mechanically abused, resulting in more than one failure. If the parts are available, a broken gear should not be too difficult or expensive to fix. A worn-out/cooked pump and motor, though, needs a new overhaul kit which supplies replacement pistons/cylinders for both the pump and the hydraulic motor, and this is relatively expensive. It is also fairly difficult to fit.
If the fluid is clean in both transmissions, it would seem likely it is a broken part in the drive train of the one that doesn't work, and with the workshop manual to hand, that should be a fairly easy fix. Alternatively of course it could be that there is just an external control that is in the wrong position, perhaps due to the linkage having fallen off, or as I said before, the transit lever could have been pushed accidentally on the inoperative transmission.
