Do I have a possible case?

My car was stolen from a repair shop Monday the 13th at 3:15pm after me leaving it for diagnostics at 10:20 that morning. The shop owner had left the vehicle unlocked. I wasn't informed till Wednesday the 15th at 5:34pm. The owner is saying he is not liable for the theft. I had signed a form when leaving my car in his care. However the form says "You will not be held responsible for loss or damage to vehicle or articles left in vehicle in case of fire, theft, accident or any other cause beyond your control." I feel that if my car had been locked then my car wouldn't have been able to be stolen. Please help