Got a temperamental civic heater - once outside temp drops to below zero the cabin heater quits. Warmed it up in a heated garage and drove it in the cold and it works fine. Let it cold soak overnight and the heater quits again.
Does your temp gauge come up to a normal level when the heater isn't blowing hot air?
Is the "heater" air lukewarm, or cold?
Are you sure you have enough antifreeze mixed in it to keep the heater core from freezing up?
I would guess you have an air bubble in the heater core, but then I'd expect it never would blow hot air. Double check your coolant level in the radiator. You might bleed the system if your car has an air bleed valve.
The temp control knob/lever is connected to valve in a hose that leads to the heater core. If this valve is operated by an electric motor, maybe the mechanism is literally frozen up with moisture and so the system doesn't allow hot coolant to enter the heater core. Or, if it is cable operated, maybe the sheath is slipping in cold temps and so the valve isn't opening.