Have you spawned in a world you don't recognize? It is possible that you have spawned in a different world and your world is still on your server.
You can check in your File Manager to see if your world is still there. If so then you can either change the Level Name under the World tab to that name or you can use the Multiverse plugin to switch between worlds.
If you are not able to find your world then it is possible that someone has deleted the world or clicked on the Reset button instead of Restart. The only way to restore your world, in this case, would be to use a manual backup from the Backups tab on the dashboard.