This is a question I see pop up a few times a week I see a lot of people having issues with the coolant buzzer going on and staying on as soon as the turn the car on and(this is key) they have verified that they are topped off at the filler neck. The most common reason for this(at least 90% of the cars that I have fixed) is the connector on the wire has become loose or disconnected close to the AC/PS/Front Rotor Housing by the spark plugs. It is a single black wire. I do not have a picture of what it looks like since I am currently deployed, but look/feel around... you'll find it. Many times if you have recently changed your spark plugs you may have knocked it loose.
I see alot of people saying to just ground the sensors out. This is the wrong answer. Take some time and find the issue. If you have verified that the connector is in fact plugged in, post here with your questions and we will do our best to help out
Brent