Why did my a/c suddenly stop blowing cold air?

Last night I was driving, and my a/c was working perfectly, the air was freezing cold. I heard a weird sound, kind of like a squeal, and it stopped blowing cold all of a sudden. I checked the belt, it's fine. The compressor is working, and the clutch is engaging. The fuses and relays are fine. My brother is my usual "backyard" mechanic, and he is baffled, we don't know what to check next. Could it maybe be low on freon? Or is there a different problem that we overlooked?