What could be causing no air to flow out of anything but defrost vents?

most likely you have a vacuum leak under the hood,passenger side,near firewall,check the smaller lines first.start the engine,turn a/c on high setting,listen for hissing sounds.may have to run your hand around the hoses to find the split,cracked or unhooked vacuum line.repair the hose.when you have a vacuum leak the blower will default to the defrost mode.