The same happened to me with a truck today. I was stuck in the middle of nowhere. It was the server where my clan is, and a clanmate was near, so I asked him to get near and kill me. But before he was able to arrive, he got a call and logged out while driving his bike. Meanwhile, I spent my time trying to move, and I finally was free. But when he logged in, he was stuck too. What I did, was take his bicy and get it near him, so he was able to call the menu and get onto it again. Maybe it could work with another vehicles too. But you'll need someone to help you. You can't even kill yourself...