ORIGINAL: Villa
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.
Thank you Villa, great suggestion. I have come into a freebie 20 gal. compressor. It must be fairly old as it has a round "sight glass" for compressor lube level. I'm going to pull it all apart and paint it. I'm going to me looking (now) for moisture and a spun con rod or main bearing. She just too old and nice a girl to trash.
Best regards,
Bill