“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.
Apple discussion on this topic