How high does the engine race when fully throttled? If engine operation is ok then it could be a few things- trans fluid level low-tranny worn out- inching spool sticking, brakes sticking or the plug in the torque convertor has fallen out.

The first thing to check is trans fluid level. Dipstick is on top of differential housing, between the dash cowl & the upright. Warm engine to operating temp, tilt upright full forward, set parking brake, put trans in nuetral, keep engine running & pull dipstick. Trans fluid level should be clearly marked. Trans fluid should be bright pink. If needed, add Dextron ATF fluid- the newest version will work fine.

The next thing to check is idle & full throttle rpm in nuetral, followed by the same check in forward, then reverse. When checking rpm in forward & reverse, make sure brakes are fully applied & wheels chocked.

Report back after those 2 checks are completed