The a/c worked when the car was new. That it doesn't work now indicates that something has changed. On cars with a constantly circulating heater there is a door that opens and closed to allow air across the heating coil. If your a/c system works but you are not getting cold air in the cap I would venture to say that system that controls the heater door is not working. In older cars this door is often controlled by a sliding lever on the dash. This lever is connected directly to the door by a cable. If you have this type system then the only think that can go wrong is that either the lever is frozen or the end came loose from the door.

In newer cars this door is controlled by a vacuum pot. If the pot doesn't work you might have a vacuum leak, a bad pot, or a plugged hose.

In even newer cars this door maybe controlled electrically. I don't work on new cars so I don't know.