If it is just your heater it is probably the air vent doors getting stuck or not opening. If it is the A/C and the heater it might be the blower motoro relay.
I am going to give you a walk through diagnostic on situations where heaters do not work. There iss an actuator that is a little motor that opens the heater doors and other doors as well. Sometimes they are not hard to get to.
Let me know what you think of thiis nad we can go from there. Just use the link below to get to the diagnostic;