Cheap compressors are oil-less. That means after about 500 hours, most give up the ghost. Personally, I'd just get another. Some things are repairable, some might be but aren't worth repiaring, and some things just need to be trashed. If the compressor is frozen, it's probably done. Perhaps you can revive it, but 15 years is a long run for an oil-less unit. Who knows the condition of the tank. Did you ALWAYS drain the air and any moisture in the tank. I doubt that, I am fussy and I can't claim yes to that question.
The more I do, the less I accomplish.