Based on the facts you state, unless they have an existing lien or an ownership interest in the car, they cannot properly repossess it. Since they do not currently have possession of if, I believe they are no longer in position to file a mechanic's lien.
The above said, they will probably come calling on you very soon to ask you to pay them for their work. At that point, you will need either to negotiate a settlement of the matter with them or hire a lawyer to deal with the situation. If you ignore the situation, they could sue you and/or make a negative credit report on you.