If the gear shifter goes directly into the gearbox through the floor, remove all the components to inspect the plastic or teflon bushing just inside the gearbox that the gear shift pivots on, it may be broken into pieces.
If your gear shift goes to some linkages, disconnect them one link away from the transmission and see if you can change gears manually from under the vehicle, this test will check for bad bushings in the linkages one link at a time working your way back to the gear shifter, dont get run over, be super safe please.
It may also be possible that the clutch master or slave cylinder is not pushing on the throw out bearing to release the clutch when hot. Check the distance of the slave cylinder travel when cold and when hot. Fluid may be bypassing a soft seal when hot.