There is a drive coupler that is between the pot and the hydro motor. It is known to strip out. Your belt that runs hydraulic pump will not be moving. It is possible for shaft going into pump to break in half. Check hydro cable is moving the arm. The arm has fine splines that shift over the control linkage and can strip out. Crawl under trans and make sure all drivelines are good and shift linkage isn't missing any pins.
Most likely if you we're driving and it just quit, your drive coupler is toast.
I have one. I have been inside and outside that entire system multiple times. There is a lot of testing to do, to just jump to replacing the pump. You need to check pilot pressures on the control valve and then at the servo pistons to eliminate the control valve. Doorknob nailed it right on the head. That inch pedal controls that control pilot pressure and there isn't any way you can tell without pressure gauges. It takes a lot of fittings and different gauges to do the proper tests. It usually takes me several hours to do it and I have done it a half dozen times.
I want to correct myself from earilier. I believe the hyd. pump belts will be turning if the coupler shears. You would have to look way up inthere and verify the input shaft of the pump is turning.