Greetings!
Doesn't sound like a very reputable shop; even in a busy time of year it is highly unusual for any collision repair facility to have a vehicle for such a significant amount of time.
If an insurance company is paying for the repairs, I would suggest contacting the insurance company for assistance. The insurer has the ability to stop payment or recover payment if the repairs have not been performed. Also, most auto policies cover rental reimbursement; insurers NEVER tolerate extended repair periods from their approved shops when they are paying daily untill the vehicle is completed.
If you are paying out of pocket, go get your vehicle and take it to a more reputable repair shop. You own the vehicle, and the shop has no legal authority (at least in my state) to retain posession of the vehicle, even if there are repair costs due. here in Pennsylvania, a repair shop has the ability to file a lien on the vehicle for repair costs due, if the customer has signed a written RO or estimate for the vehicle, but we have no legal authority to prevent the customer from taking posession of their own property. Remember, YOU own the vehicle, not the repair shop.
If some repair work has been done to the vehicle to this point, you MAY be responsible for the cost of that work, but certainly not the total amount estimated; this would be a matter for a small claims court to resolve.
In the meantime, go get your car and have it repaired elsewhere. I would suggest taking along the title to the vehicle and a local police officer or constable to keep things peaceavble if you suspect that the shop will attempt to refuse to release the vehicle for non-payment. Obviously, you don't want the current faility to complete repairs on your vehicle in any case now that the business relationship has soured; such repairs would possibly not be up to standard now that they have no interest in keeping you satisfied. Doesn't sound like a very professionally operated place anyway...
LAws regarding such property and business rights vary somewhat from state to state; you may wish to schedule a consultation with your attorney if you have any specific questions regarding your rights in your state.
I hope this is helpful to you; if so, an accept would be most appreciated! Thanks!