If he is only co-signer on the financing, not a co-owner of it, then this is theft: as a non-owner, even one who has guaranteed the obligations, he has no right to it. You can report it as stolen and/or sue him for its full value.If he is a co-owner, too, then this is not theft: you can't steal what you own. But if he is depriving you, the other owner, of use and access, this is a civil dispute over the use of property: you could sue him for a court order requiring him to make it available to you, too, or (and this would be easier) for your share of the value (50% of the price, basically) since he has deprived you of that value.