Under bailment law, the mechanic had to exercise reasonable care of your property while it was in his possession, and he's liable for damages he caused through his negligence. Have you asked him to report this to his insurance carrier?
I'm only licensed in CA. Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
When you take a car to a service garage for them to look it over or perform service on it, in most states you create what the law calls a "bailment”. That requires the shop to take reasonably good care of the car while it is in their custody. If they don’t, then if the vehicle is damaged while in their possession and control, you have a right to recover damages for what they do wrong or the damage they cause (or, if they don’t return the car at all, for the value of the car itself). But you have to be able to prove that the damage occurred while the garage had it and that they did not take reasonably good care of it. In other words, you have to be able to show that it was basically the fault of the service garage that it got damaged. The one exception to that is under a bailment contract, like you have here, where they return the vehicle back to you and it is not in substantially the same condition that it was in when you gave it to them. Under that circumstance, in many states the service garage may have the burden of proof to show that they are not the reason for the damage. Because the law is different in each state you need to talk to a local Consumer Law attorney. Call your local attorney's Bar Association and ask for a referral to a Consumer Law attorney near you or you can go to this web site page for a Free Online 50 State National List of Consumer Law Lawyers (http://www.ohiolemonlaw.com/ocll-site/ocll-locate_local.shtml) and find one near you (lawyers don’t pay to get listed here and most of them are members of the only national association for Consumer Law lawyers, NACA.net). But act quickly because for every legal right you have, there is only a limited amount of time to actually file a lawsuit in court or your rights expire (it's called the statute of limitations), so don't waste your time getting to a Consumer Law attorney and finding out what your rights are. If this answer was helpful, please give a “vote up” below. Ron Burdge, www.OhioConsumerLaw.com