The quickest way to get your car back may be to file your case in small claims court to get it before a judge for a court order, if the car and your damages are under $10,000 in California. You will have to first send a demand for return of the vehicle in writing to the shop owner and send it by certified mail, return-receipt requested so you can prove he received it. Be sure to keep an eye on the potential lien sale date so you do not lose the car in a sale. Good luck.
The above answer is for general information only and is based on the information you posted. Every case is fact dependent, so to get a thorough analysis of your situation, you will need to consult face to face with an attorney licensed to practice in the jurisdiction where the incident took place. Do not conclusively rely on any information posted online when deciding what to do about your case.