I was driving my truck to a friends house. The temperature gauge got up to the top temp. I slowed down and the gauge dropped all of a sudden back to the bottom. I continued to drive. When I got to the house, white smoke started to come out from by the engine. I turned off the engine and let it sit for a while. It was leaking coolant but their was still plenty left in the reservior. It then wouldn't start. What is the problem and how much might it cost to fix?