u mean the coolant level in the recovery tank? u can start with the thermostat, it maybe stuck closed. u can pick up some coolant dye and add that to the radiator to see where its going if its leaking. maybe also be a bad radiator, water pump, hose leak..hope its not the head gasket.

they tend to leak at the neck of the radiator at the hose to the recovery tank. the tank splits off of the back. its plastic and it cracks. if u can move the radiator back and forth and its loose that is a clue its going bad.