Are you sure your whole server reset and not just the world? Did you change the value in the level name box on the panel? If so, your world should still be there. Check with the /worlds command. That just sets the default world of your server so if you changed it to a name of a world that doesn't exist, it'll create a new world and put you in it. Just change it back to what it was before, click save, and restart the server.