Yes, I also suggest to check the coolent level. Then check the heater valve, if it not open then hot coolent cannot enter to the heater. You can chect it but unplug the wiring and give it 12 volt power, and see wheather it work or not.

If not it's problem, check the potential meter of your temperature control, as mind have problem few days ago, you can try to clean it, and see wheather it better or not.

If not as well, try to test the temperature sensor.

If your test everything and can't find anything go wrong, then, might be the ECU causing problem.