Because the heater is getting cold, that indicates low coolant. It leaked out and the leak needs fixing, but as soon as the engine cools off, refill the coolant and keep it full. Never add coolant to a hot engine without the engine running. Your electric fan might not be working. The fan moved air through the radiator and air circulated the same way when the car is moving. Check that.
If you can see your water pump, feel around the neck of it for moisture. Any moisture there means the pump leaks and needs replacing. Some cars have the water pump behind the cam belt. You can't get to it without a lot of hard work- get that done by a qualified mechanic if you can't find your leak elsewhere, such as a leaking hose or bad radiator cap.
If your electric fan doesn't work, that might be the whole problem. Usually the fan relay fails before the actual fan. Replacing the relay will solve the problem if that's it. Good luck. China OBD2 car diagnostic tools wholesale on CareUCar