If your cars overheated and lost some water then your heaters won't work. The water level needs to be full for the coolant to circulate through the pipes which transfer heat to the interior heater. If you haven't already done so just top your coolant back up to where it should be and you shouldn't have any problems. If you've already done this then I don't know what to suggest except the heater bypass valves that i've heard can become faulty and cause your heaters not to work. Replace them with copper piping and you should be away!
hope this helps, Acko
