They can do nothing if you have nothing.They could put a lien on anything you own free and clear.It will lower your credit rating but better for you to return it voluntarily.The bank will sell it at auction if no one takes up payments and they will try to get you to pay the balance.
If you don't own anything free and clear they can't do anything.The bank can sell the note to someone else and they will try to scare you into paying the note.Talk with a lawyer or your Justice of the Peace.