I had the same issue when I was in an accident. Car was towed to some sketch shop and the owner wouldn't give the car up until I paid a $2500 storage fee after two days. My insurance company called the police and they dealt with it by sending a sheriff down to TAKE the car.
The body shop can't start repairing the car without your signature. My rental car was even tied in with this scam and the body shop copied my signature from the rental forms for the work order to start the body repairs. Luckily at the time the car was in my mothers name
In the end my car was taken to European Autobody and I didn't pay a penny