Depends on what the problem is. Does it still have a charge? Is it just low? Compressor work? Try jumping the low pressure switch connector to engage the compressor for a second or two to see if it works. If it doesn't, then more troubleshooting will be required. If it's just low or empty, where is it leaking? Might have to use a leak detector to find the leak given the time passed and any UV trace dye is probably gone.
I have had it filled twice in the last few years and seems to go warm within a couple weeks. I am guessing its probably a leak somewhere. Not sure if it has developed into anything more because it hasnt been used in a few years.
It's possible that air and moisture got in but hopefully the compressor is OK. Probably need to be flushed of all the old oil as it is hydroscopic. Likely they'll add a bit of R134a and look for the leak using a tester. It could be anything from a seal to the compressor.