My guess is the air end is locked up. First thing is drain all of the water out of the system. I suppose it is possible that water may have even backed up into the air end. Moisture is a common problem in air compressors. Then try to turn something; the motor, the air end, the coupling, to check for freedom of movement. If the running gear is lubricated, oil or grease, it may need some. I designed and trouble shooted larger air compressors for some 30 years and they all have similar problems.