Has this message popped up for you?
This message appears when the tablet or device has WiFi or mobile data turned on, but the internet is not properly connected - e.g. there is a problem with the service, or a poor quality data connection. Although showcases can be viewed offline, a working internet connection is needed to sign in.
You can always try the old turn WiFi off/on again, or connect to a different WiFi connection and try again.