Heater is blowing cold air

Check the following:

Is coolant level full?
Is engine warming up to normal operating temperature?
If the two aforementioned items are good, allow engine to warm up with heater on. Feel both heater core hoses. Are both hot? If so, suspect a defective blend air door. If only one heater hose is hot, suspect a plugged heater core.

This may also have a heater control valve located on either heater hose. If so, check the heater hose just before and just after this heater control valve, both hoses should be hot. If the hose going into that valve is hot but the heater hose coming out is warm or cool, then look to see why that valve is not opening, maybe a vacuum hose fell off of it, etc, of the valve is simply stuck.