Engine Exhaust System problems of the 2002 Ford Econoline

Took the van to the store. Upon returning from store started the van, there was a loud explosion as if a bomb went off. Everyone looked because it was so loud. Turned the van off and restarted it. It sounded like a drag car at the track, no exhaust system at all. Took cow off and looked inside, found the spark plug with the boot laying on top of the valve cover or intake, not sure there is no room and just enough to get two fingers in to retrieve it. I had noticed a little ticking earlier in the day like an exhaust leak, but did not find one. Had vehicle towed to repair shop, where it is now. Not sure what will happen next.