First things first...make sure the radiator/coolant is full. Second...make sure your thermostat is working properly, as if it isn't, it would stay in an open position and not allow the coolant to be heated properly. Check the temp gage and make sure temp is in the middle after warmup.
Is the defroster hot/warm? If it is, the problem may be in the climate control...but check the simple things first. Good luck.