Remove the needle and unscrew the head so that there is nothing but a hole in the front of the airbrush (don't lose that Teflon washer that goes between the tip and body!) and see if you are getting air. If you are it's something in the tip that is plugged up.
If you don't get any air that way then it's something from the air valve back. Make sure the trigger is properly seated on the tip of the air valve, make sure the hose isn't kinked. Disconect the hose from the can and crack the valve slightly to make sure you actually have pressure in the can.
If none of those fix it, it's probably something in the air valve itself. The air valve can be disassembled and replaced on a part by part basis, but I usually just replace the whole thing.