Vehicle contract

What can I do if I gave someone written consent to drive my vehicle and they will not return it to me.It was being financed and I let them take over payments on it.The contract says if they didnt make the payments on the 11th and 25th of the month I would notify the finance company and the vehicle would be repossessed.and that I was not responsible for any money that was lost.I tried contacting the people but there phones are off.I went to the address they provided and theres nobody there.Im not even sure if the id she provided is real or fake.The vehicles registration and insurance is expired.I believe they are hiding the truck some where so i can not find it please help.Thank you!