Turn off the Automatic Backup option if you can under Settings>iCloud>iCloud Backup.
If that doesn't work, try resetting the device, then turn off Automatic backups until you can figure out why it isn't backing up. To reset: Hold down the Home and Power buttons at the same time and continue to hold them both down until the Apple appears (up to 30 seconds). Once the Lock Screen displays, see if you can get into Settings to turn off Automatic Backups for now.
Cheers,
GB