Have no heat

First of all, does the temperature gauge indicate the engine is reaching normal temperature? If it is, once the engine is warmed up, pop the hood and feel the two heater hoses. They should be too hot to hold onto for very long. If they are not real hot, but the upper radiator hose is, the heater core may be plugged. Flushing it with a garden hose usually clears the blockage.

If the heater hoses are nice and hot, the problem is related to the heater controls or an actuator. Diagnosing that can get rather involved and is best left to a mechanic.

I need to know if the temperature gauge does whatever it does when the heater was still blowing hot air, and if you felt the heater hoses. If the gauge does not work, how will we know if the engine is not getting up to normal temperature or if it is the gauge that has the problem? There could be two unrelated problems, but we need to be able to make some observations and find some clues.

You can also have your mechanic connect a scanner to view live data. That will show the actual engine temperature from a different coolant temperature sensor. One way or the other, we need to know if the engine is getting hot enough. If it is not, there is no point in working in the heater system.