Gearbox faulty

I assume this has a shifter cable. See if perhaps the shifter cable broke or cam off at either end. Find where it connects to the transmission shift lever and watch to see if the cable and shift lever move while someone else moves the shifter through all of the gears.

If the shift lever is moving and the transmission is full or good clean fluid, either one of three things is happening. The transmission filter is clogged with debris that has fallen off clutch packs from within the transmission and in that case the transmission will have to be either rebuilt or replaced. The front pump has stopped working in which case again the transmission will have to be either rebuilt or replaced or simply the shift control shaft has come loose for some reason from the manual shift valve. To see if that is the problem or if the filter is clogged the transmission cover will have to come off. As far as the front pump, there is a pressure test for that and a test plug that is removed to hookup the tester. Below shows that gauge hooked up and in drive idling the pressure should be 58-66 psi, in reverse 117-128 psi.