I'm assuming you meant temp sensor. Have you checked the radiator resevoir? If it is empty you should add some to it.
Check if the heat works. If it doesn't it means you are short on coolant (assuming the car is already warmed up).
To answer the question, either the sensor is bad or the car is overheating. Sensor is bad, not that big a deal (you will lose mpg as the car tries to compensate), car is overheating, very bad, it will eventually lock up (new motor, at least same price for repair).
Hope this helps