If you're receiving error messages when the game is trying to start up, the first thing to try is closing all apps on your device and disconnecting any internet and reconnecting it to see if that kicks it back into gear. You can do this easily by turning Flight Mode on to disconnect any internet connections and then off again to reconnect.
If this doesn't work, try restarting your device. Sometimes cached data can build up over time and causes issues with your apps, so a device restart can clear that data and give your apps a chance to start fresh.
If these suggestions don't work, please tap the 'Contact Us' button at the top right of your screen to send us a message!