are you running low on water? if it wont go of and your low on coolant fill it up with the engine running let it run a little put some more in but dont over fill it, turn it off give it a minute or 2 start it up check your water level again if need be add more.. if the light stays on you've either got a hugh whole somewhere witch will be evident buy the huge puddle under the car...if none of the above help then you must have a dodgey sencor most likely in the expanstion bottle or something..
hope it helps................dave