1999 Ford Expedition On occasion it won't start

Engine problem
1999 Ford Expedition V8 Wheel Drive Type unknown Automatic 83000 miles

On occasion the car will not turn over. Last week it wouldn't start after I got back from an errand. It sat for a couple or three hours and it then started. I saw a little bit of white smoke, but I think it was because I had flooded it. The truck had sat for 3days without being started. Never overheated or stalled before. Up to date with maintenance and gages are normal. What could be the problem. This is intermittant trouble.