the controls for the heater are defective or the heater core is disconnected
or a bit more information might be helpful.
o.k., so the gauge works, was that what you were trying to determine in the first place? Can you feel heat in various hoses? the bigger ones that go from the engine to the radiator? the smaller ones that go to the heater?
What happens when you move the heat control lever?
If you are looling for heat to come out anywhere how about does it come through the defrosters?
Does the car have A/C?, Does it work?