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?
It sure sounds as though your engine overheated. Possibly due to air in the system, a stuck thermostat, or something that resolved itself as you continued to drive.
But the damage may have already been done and the white smoke, which I assume was water being drawn into the combustion chamber, indicates that your cylinder head may have warped. This compromises the seal between the head and the block and allows water to be sucked into the cylinders. Note that even if the overflow reservoir is full, the system itself may be low or even empty.
Worst case here is the need to replace the engine, a relatively expensive proposition.