First run the car to normal operating temps. Then feel upper and lower radiator hoses. If one is hot and the other isn't, you have a stuck thermostat. If both are hot, then turn heat to high/hot and rev the engine. If it blows hot air while engine is reved high, then there's air in system (How air got in system is another test).
If it does not blow hot air, feel both hoses (smaller than radiator hoses) going into and out of the firewall. If one is hot and the other isn't, then either the heater core is clogged or the heater control valve is stuck and needs replacing. Flush system first.
If all of these check out fine, then your problem is either a stuck blend door inside the HVAC unit or you have a vacuum leak/faulty solenoid also inside the unit that needs replacing.