Was the Auto Body Repair Shop responsible for my spark plug blowing out?

I was involved in an accident where someone had side-swiped my vehicle, a 94 Honda Del Sol. Geico referred me to an auto body shop for the repairs. After two weeks, they called me to come pick up the vehicle. Repairs looked great, however, as soon as I drove it off the lot, there was an immediate difference in the way my vehicle drove. Hesitating, sputtering, not wanting to get up to speed, etc. I turned it around at once and took it back. The technician shrugged it off saying that it was an "old vehicle" and that it drove fine once warmed up and sent me on my way. The next morning on my way to school I heard a loud POP and rattling from under the hood, pulled off to the side of the road only to discover that a spark plug had popped out and busted my plug cover beyond repair. When I dropped the car off there was no evidence of a loose spark plug, it purred like a kitten. So I was wondering if there was anything they might have done to it at the shop. My vehicle was additionally left unlocked (by the shop) in an unsecured, unmonitored parking lot for days at a time, so it could have been tampered with at anytime.

No, body shops couldn't care less about your engine and wouldn't have gone near it. The car is a 21 year old Honda that was no doubt popular with 20 other young owners that thought they knew what they were doing and stripped out the cylinder head when doing one of likely many tinkering tune ups.
Not what you want to hear I know, but very likely the case.

Nope, a body shop wouldn't have any business messing with your spark plug

NO, the COLLISION is responsible for the engine problem. Body shops don't do engine work.