If you log in to the game to find your characters missing, you may have simply logged in to the other regional megaserver. Logging into your original megaserver (North America or Europe) should reveal the missing characters.
To do this:
Xbox One
Launch the game application from the console's main menu.
Press [A] to start the game.
Under the Game Startup menu, go to Server Select instead of Play.
Use the [D-pad] to select North America or Europe as the regional megaserver.