Does the power steering and lifting work ok? If so, the drive motor may be so dirty or gounded that it won't allow it turn on. You should unplug the battery and check the resistance from the power plug on the forklift to the frame, both positive and neg. It should be at least 100,000 ohms but if it gets down below about 20,000 ohms it won't turn on the drive. If that shows a ground, you could try washing the motor with electric cleaner and blowing it out.