My car's heater is not blowing heat what can it be

 

You either have a blockage in your lines that feed the heater core, your mix cable/valve is stuck, or your thermostat needs to be replaced. I had the same problem and it was my thermostat.

 

Is the blower motor not even forcing air? Reach under the center of the dash with the heater turned on full blast - is the heater core even getting warm? You can stick your fingers up inside the core under the center of the dash.

Is the car swapped? Sometimes people bypass the heater core hoses on the engine.