If you are using android, i suggest the following:
Search Google Play and see if you need to update the game app, if it's not that, uninstall the app, re-download game app. If that doesn't make it work there is 1 more thing you can try
Clear game app data
Clear Google Play app data
Clear Google Play services app data
Uninstall game app
Shut down everything else running (current programs so nothing is running)
Reinstall app.
That's every trouble shooting method I have ever had to use to fix issues with any app.
If you are an iOS user, i cant help ya. I've never been a member of the evil empire.