This is most likely due to your app signed out from your Gamecenter or Google Play account, or signing in.

 

Please first try logging in or out of your Gamecenter or Google Play account on your device.

 

If this doesn't work, Customer Experience can help you retrieve your original account if you remember your original account name or Profile ID. Please submit a ticket with this and all other relevant information and we'll be happy to try to help.