I'd say that your friend is going to have to pay you for the value of the car, whatever that might be, because the situation, with your car in his back yard, created what the law calls a bailment, and that makes the car's safekeeping his responsibility. He had the right to end the bailment, on reasonable notice; what he did wasn't even remotely reasonable. It was one thing to push the car out of his yard, but leaving it unlocked with the keys inside is unjustifiable, in my opinion.