This is a known issue that will most likely be fixed in A20.4. To fix it now, start loading in the game. Before the game finishes loading, hit F1 to bring up the console. In the console type: tp 0 0 0 Then hit enter. If you timed it right, when you finish loading, your character should be at map center. You can then make your way back to your bike either by teleporting or just running there. To prevent this from happening, never leave the game while on a vehicle or next to a vehicle. Get off your bike and pick it up. If you you have too much inventory, drop a box and put it in there.
