A/C System

I own a 94 Buick Century and the problem just popped today. I was out doing my daily rounds and had the A/C running, it worked fine up until I headed to Wal-mart. Before entering, it worked but when I left the store, it was no longer running. I can hear the hiss of air escaping when I try to turn the system off and on, but it sounds like the fan isn't running. What's wrong and can it be severe? A reply ASAP would be greatly appreciated.