This could be a problem with your blend door. They can break inside of your heater box and not allow the heat to pass through the vents. You can check the heater core hoses in the engine compartment at the fire wall to see if both hoses are hot. If they are hot the problem is in the heater box. Also check the blend door actuator to see if this is working. If the blend door is broken this repair would require removal of the dash to access the heater box. Our shop would charge $800 for this repair.