Answers to appengineer's questions, beyond, "the hydraulics don't work" would be a big help to solve this mystery. Having said that, you can reduce this to two elements: electrics or hydraulics. Operate the truck controls as if lifting while checking for voltage at the motor terminals. If motor runs without lifting, the problem is hydraulic. If voltage appears without motor operation, then safely apply battery voltage directly to the motor terminals with cables. If motor runs, check control circuit for cause of insufficient current to run the motor.
This only a start, but it should get you close.