Tampering with car

I took the car in to have a rear bumper repaired at Fitzperfect Autobody. Two weeks before it went for this repair, the car had a major service done. After completion of the repairs, I collected the car from the panelbeaters on Friday 9th October 2015.Less than 5km after collecting the car, it didn't want to start. I phoned the panelbeaters who told me that the mechanics of the car is not their problem. The car had to be towed away.