http://www.craigcentral.com/models/trouble.asp
You either have a paint or air blockage (obvious but true). If you have airflow then the problem is in the paint area. If you don't have airflow the problem is in the air area.
Take the tip out and look through it. If you can't see light through it then it is plugged. There should be a group of holes around the nozzle (at least Badgers have them). Make sure all of those holes are unobstructed (run a thin piece of wire through them if necessary). Make sure the vent hole inthe pain cap is not plugged or covered by the seal in the cap. Disconnect the airhose from the airbrush and make sure you have airflow there.