a/c suddenly blows warm air

My a/c has always been very good, except once when it went from blowing cold to hot fairly suddenly. It turned out that someone had stolen the a/c relay and simply replacing it did the trick. This time I've checked and the relay is still working. I can hear a click near the compressor trying to get it to cycle on, but it doesn't come on. Is there an inline fuse that is blown our something else that may be preventing the compressor from coming on? The belt and pulley are both turning.