First things first. Check that the coolant level is good. There is an air pocket in your cooling system or the heater core. On the hoses to the radiator there will be a small screw valve that you can use to let the air out. With the vehicle running and warm, loosen the screw and let it flow until there is no air coming out. Allow the engine run for ten or fifteen minutes. Loosen the screw again and let any air out. It may take several times. Also, make sure your heat is on while doing this. That will push any air out of the heating system. If you cannot get any air to come out and this doesn't fix the problem, then you can start checking sensors and gauges and such. Hope this helps.