“iPad not backed up” message blocks restart
One morning, on turning on my iPad, I was presented with a message saying “iCloud Backup: This ipad has not been backed up for 2 weeks. Backups happen when this ipad is plugged in, locked and connected to Wi-Fi.”
Hitting the OK button did not have any effect, so effectively I was locked out. Perhaps it was repeating the same message, or perhaps it was just frozen.
Holding the Sleep button down also did not fix the problem. I had forgotten the magic reset button combination: hold the Sleep and Home buttons down simultaneously for several seconds until the screen goes black, and the Apple logo reappears.
Resources
Apple discussion on this topic
Apple discussion on this topic