You may need a new thermostat, this can cause the heater to blow cool or not very warm air. If it is just cold air coming out and no heat at all, you could have a heat door on the heater not opening or a water shut off to the heater not opening. It could be a plastic gear stripped and not letting the heat door open. Also could have had something fall down the defroster vent like a pencil or something and blocking off the door and it won't open.
