Something is obviously blocked, either the air path or the paint path.
Make sure you have air, if not disconnect the hose from the airbrush and see if you have air there. If not track backwards and find out why you don't. If you have air at the hose end but not when you push the trigger, make sure the trigger is properly seated on the air valve and make sure you can feel the air valve moving when you press the trigger.
If you have air but no paint, make sure the vent hole in the cap is open, the tube and cap head are open, etc.