This game has a few bugs which can make your vehicle disappear.
It is also good practise for servers to remove vehicles on server restart.
Loosing vehicles is one of the most annoying things in this game, for players & server admins.
To solve this issue we created a vehicle garage mod.
We could make it a real garage from the interface side, but we like to keep this system simple to use for players.
When you place a vehicle it stores it inside a database including installed mods (fuel is not saved for balancing reasons in PvP).
You can use /vehicle in chat to restore you vehicle at any time. Even when it doesn't exist anymore due to bugs, server restart, crash or when destroyed by other players.
It is highly recommended to not pickup your vehicle before you leave the server or it restarts. Just disconnect and use the command when you come back! This also gives you a free inventory slot and others can't steal your vehicle anyways like this.