2001 Ford Windstar heater not working

On my 2001 ford Windstar, the heater is not blowing warm air all the time. Iwas told it could be the thermostat, I checked the coolant resivoir, it was close to empty, so I filled it with water, the heater began to work right again. That was 5 days ago, this morning it did not want to heat up at all. I checked the resivoir and it is almost empty again. Would changing the thermostat fix this problem and why is the resivoir empty. There are no puddle's under the car where if could be leaking, and it is not running hot either.