[first time answering questions] car heater works like the reverse of the normal cooling radiator - in fact the heater is like a small version of the rad that's on the frnt of the car - the hot water...
I would check the coolant level first. When the coolant drops below a certain point,Warm coolant that makes the heater work is no longer present and all you get is cold air. If this doesn't work, I'd...