The 20 GB complimentary storage upgrade you received when you moved from MobileMe to iCloud has expired.
Your iCloud storage has been automatically adjusted to the free 5 GB plan. You are currently using 16 GB of storage.
For more information, see this article.
The iCloud Team
If the storage you're using in iCloud exceeds the total storage available, iCloud Backup, Documents in the Cloud, and iCloud Mail will temporarily stop working. To resume using these features, you can either reduce the amount of iCloud storage that you're using, or purchase a storage plan with a larger amount of storage than you're currently using.
You can choose a new plan by going to Settings > iCloud > Storage & Backup on your iOS device.