[fontgeek]

Remove the airbrush from the hose, and make sure you are actually getting air through the hose. If you are getting air through the hose, then proceed to the next step.
Remove the air valve from the airbrush, and screw it back onto the hose, then, with the air compressor on, push down on the tiny pin that opens the airvalve. Are you getting air coming through? If not, then you have some kind of obstruction in the valve that came through the hose. If you are getting air coming through the valve, then re-attach the airvalve to the airbrush, and proceed to the next step.
Remove the needle cap, the nozzle cap, and the nozzle itself, can you get air coming through the brush whithout the nozzle cap on the brush? If you can, then your obstruction is in the nozzle cap itself. If you can't, then you have something blocking the air passage inside the brush itself. If you have an obstruction in the brush itself, then you can try removing the airvalve again, and disassembling the rest of the brush, and let the body of the brush itself soak in an appropriate solvent, then try blowing the obstruction back out the air passage and out of the opening that leads to the air valve. Since the blockage obviously won't come out the front end of the brush, the easiest place to remove it will be through the hole or opening it entered by.

If the obstruction is in the nozzle cap itself, then soak it in the appropriate solvent and try blowing the obstruction back out. You may find that a fine bamboo sliver or toothpick will help remove any foreign substance.

Keep in mind that the air doesn't normally go through the nozzle, when it does you will see paint bubbling up in your paint cup or jar/bottle, If you aren't getting air out anyplace, then you have something obstucting the normal flow or path, or, you have a compressor or airsource that is no longer supplying air.
Good luck, and let us know how it turns out.