If the you still get air flow, and it's just not warm then it sounds like the heater valve cable came off or it's not opening. When you turn the heat on the switch operates a cable that opens the valve, that lets warm coolant flow into the heater core, then the blower fan blows air over the heater core (making it warm), and finally out the vents. So it sounds as if the valve is faulty or the cable came off.