I assume you were originally a MobileMe subscriber who migrated to iCloud? MobileMe subscribers who migrated to iCloud received complimentary extra storage up until September 30 2013. This has now expired, as was explained in emails sent out prior to the end of September.
If you have less than 5GB data in iCloud then your storage simply reverts to 5GB.
However if you have more than 5GB data, Backup, Documents and Mail will stop working until you either delete sufficient data to bring you within 5GB or purchase additional storage.
For instructions on purchasing additional storage please see
For help in reducing your iCloud data please see