2004 Volkswagen Golf Failed Oil pump

I have recently been involved in a car wreck, the damage nearly totalled the car, but was just about repairable. The car has been maintained regularly and had an oil filter change etc in March. The car was perfectly fine before the accident, but when I travelled 5 miles away from the garage after it had been in for repair for over 4 weeks, the car stopped. I was originally told that the timing belt had been damaged, and caused damage to the engine, but I've now been told that the oil pump has been damaged (somehow), and I need a new engine. Could a heavy collision to the side of my vehicle have caused the pump to become damaged in anyway, or could the garage have knocked or damaged the pump so it stopped functioning the way it's supposed to? I have no idea whether the vehicle was used during its time at the repair shop, they're not telling me anything. I'm ultimately holding them responsible because the car was absolutely fine before the incident, and travelling 5 miles before the engine cut out tells me that there was a problem with the car that they should have spotted and so should never have released it to me (surely they have a duty of care to help me?), But they're telling me that I have to prove that the damage was caused as a result of the accident - shouldn't they prove to me that it wasn't!