If you have a social network account connected, you can login through the “Social Media” menu in the game, just use the icon of your social network of choice.
If you’re playing with no connection to the social networks on iOS, you can reinstall the game, you should make sure you’re using the same Game Center ID as your account and then log in from the start screen through “Start game” button.
If you’re playing with no connection to social networks on Android, log out from the “Social Media” menu, close the app and kill the process, then go to the system settings and clean cache for both game and Google Play Services. Then start the game and use “Start game” button.