So the engine is overheating? If it isnt, there are generally 2 reasons why a car doesn't blow warm air out. The blend door located in the dash actuates the air coming out of the vents being hot or cold. Sometimes it gets stuck. The actuator motor or cable breaks and you wont have heat. The second most common issue is the valve that lets hot water into the heater core. Sometimes this valve gets stuck and won't open because of a broke servo motor or cable that actuates it. There should be 2 coolant lines that go into your passenger side firewall and you should find the valve in that area in the engine bay or just on the other side of the firewall.