Hi, kimIN.
Thank you for the question. Try closing all open applications in multitasking and power cycling the device. Once this is done test the results. If the issue persists, I have included a couple troubleshooting articles that I would recommend going through.
iOS: Force an app to close
http://support.apple.com/kb/ht5137
- Verify that you are connected to the Internet.
- If you are unable to create or sign in to your Game Center account from within a game, try creating or signing in to your account using the Game Center app.
- Try signing out of your Game Center account, then sign back in. If you can't sign in to your Game Center account with an Apple ID, try resetting your password or using another email address. To manage your Apple ID account, go to My Apple ID.
- When using a Wi-Fi connection, verify that your Wi-Fi router is configured for Game Center.
Using Game Center
http://support.apple.com/kb/ht4314
FaceTime, Game Center, Messages: Troubleshooting sign in issues
http://support.apple.com/kb/ts3970
Cheers,
Jason H.