hmm, I've used an air compressor before to blow off certain things, like the dust from a board if it's been sat on the side for ages, but you have to do it real carefully, with like soft busts of air...
anyway, that aside.
using compresed air, (whether it's from a can or a compressor) is never the best idea, ideally you should get a good mini (like the keyboard vacums) hoover).
dust is conductive. so if by blowing around in the case you've blow a fat chunk of dust onto something thre is a good chance that you may have broke it.
IMO it's almost impossible to clean a PSU by blowing air in the vents, so try testing this first, but then as suggested above just start testing things until you find out what it is that's broken.