Low coolant in the overflow bottle. Add water / antifreeze and the light should go out. Just need to figure out where it went to in the first place though. Ceck your radiator at the same time to be sure that you do't have a coolant leak problem.
It is the dreaded BHG... i put my hand behind the exhaust and it was pretty moist, indicating my coolant is going thru the motor out the back. Sadly i gotta find the part now.
EDIT: Sorry i did not specify, i already tested it and it tested alright. I went to the exhaust and i felt the moisture, then i looked under the car and some coolant is leaking out of my exhaust piping, i have a hole in there now :hammer: