Check for vacuum just like the previous poster said. Then you're ready to check the vacuum to the system inside the car behind the glovebox.
If it quit suddenly, you're likely to have a vacuum problem. Leak in storage container in engine compartment, hose off, hose burned in two. If it occasionally did this by blowing on defrost only and didn't start suddenly, you probably have a vacuum problem in the programmer inside the car.
I used the method in pictures in one of the links in earlier posts. I used gasoline (1/4 inch) hose--about 2 inches per connection. Not pretty as the one linked, but free from friendly, local NAPA store. I bypassed the little block connector in programmer where the plastic material softens and sucks together blocking the vacuum through the hoses to the other vacuum motors that move the vents in the heater box to direct air. Usual first symptom (on dual auto air units) is that the door to make air come out dash ducts for air conditioning won't work; the air just goes on up and out the defroster instead of in your face out the dash vents.
Dual auto air on both