Packasmacks

In most cases, hitting the vehicle once with an axe or bullet will sync it to the server and allow you to repair normally.

This happened to my friend not long ago. I had to shoot him in the foot with a Makarov and bandage him. After that, he was perfectly fine, and the Makarov shot only took around 600 blood. If you don't have a friend to kindly shoot you, you could just have a zombie beat on you until you start bleeding and then bandage. Kind of risky, but better than dying a slow death.