This has happen to my Navigator twice. I know this sounds strange but it was the security system that locked the entire system. When you would turn the key just a click then nothing. Triple A insisted it was the battery which it was not. We had been in the car with the engine off and the radio on and then when we went to start the car nothing. The solution that worked for us was to close all the doors from the outside and lock them with the key fob leave the car like that for a few minutes and then using the key fob open doors and start the car.