First of all, is this shop close to home? Is it still in business? If it is, why haven't you gone down there in person to see what is going on? Calling the police will do no good unless you are there. They do not take sides, they only keep the peace if the discussion is getting out of hand. It is a civil situation unless you can show otherwise. What is the shop owner going to say? Are they demanding more money? If its gone beyond your budget, they should have just released the car to you where it stood. Did the car get stolen?
The only remedy is to get a lawyer and sue. Small claims court can take jurisdiction, but it only covers a maximum amount. If its beyond that, than get a lawyer. Sorry, this will cost you?
My advice is take the first step and talk to the owner in a peaceful manner and in person. He/she has all the cards. They can claim shop lien for unpaid time etc. Its all about negotiating a peaceful outcome.