Is your compressor belt-driven? This sounds gross, but it happened in our shop last Saturday. A rat crawled into the warm engine compartment, and fell asleep across the A/C belt. When the driver cranked the unit up in the morning, it caused the belt to jump off the pulleys, with the added joy of cutting the rat in half. Without going further into messy details, I removed the rat, and installed a new belt.
If that is not the case, you may have a line off at the compressor, a drain that didn't close all the way, or a couple more minor issues, before you get into the stuff you wouldn't want to tackle on your own. Good luck with it.