Find the two 1" diameter heater hoses running to the heater core from the engine compartment. Do they feel hot with the engine running? That will tell you if the heater core is getting coolant from the engine. If they do, then you either have a bad heater core or a stuck blend door. Shift the temp control from hot to cold - can you hear the blend door moving? If not, it might be stuck in the AC position. If the blend door is stuck the most likely culprit is a bad blend door actuator.
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