Known issue: all vehicles intended for players (currently Niva 4x4 in version 1.0.150000) gradually "sink" underground. It seems like it happens with every server restart. In a few days if left untouched, it can go as deep as half way down. When you get into the vehicle driver's seat, the vehicle "jumps out" of the ground, and usually kills the player. Depending on how deep it was, the car gets either damaged (and gets a leaky fuel tank) or instantly ruined.
Workaround: if you have another car (or know someone who has), just drive by it as close as possible (about 1 meter from it or less). The car pops out of the ground on its own, without any damage and becomes safe to enter.
If you don't have another car and really-really want to build one... Just destroy all the sunk cars you come across by shooting into its engine compartment until it starts smoking. It will despawn in a matter of minutes. Then, after server restart, the cars supply on the server will get replenished automatically, however they will spawn in random locations. So you can scout for the fresh new cars (have a few friends scattered around the map to make it easier).
