so if there is no water inside (u are in safe side ) , the next step is to check all coolant lines (check also the lines connected to the engine ), maybe the lines is not tied or damaged coz of heat,
if the heater is not working that's mean no water go to the heater core then so the heater core is fine , and the valves needs clean and it lines must be checked
ur problem is small don't worry just leaks on pipes , good look