If you find that the game is stuck on a black screen and not loading in to game then please try the following steps:
1. Make sure the game has been force closed so it is not running in the background.
2. Go to Settings.
3. Scroll down and tap on Game Center.
4. Scroll down and "sign out" of Game Center.
5. This should have signed you out, if you look in the Game Center section it now has a switch that is in the off state.
6. Launch the game.
7. When asked to sign in to Game Center select cancel.
If the game launches successfully please re-launch the game and sign in to Game Center.
If you found that Game Center was not accessible in the Settings, please try hard resetting your device and following the steps again.
If you are still having issues please fill out a bug report here.