The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content. (rsReportServerDisabled)

Reporting Services might encounter the below error, suddenly even though it was working perfectly. There can be many reason for this.

The report server cannot decrypt the symmetric key that is used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content. (rsReportServerDisabled)

The Symmetric Key might have got corrupted.

The only way to resolve this is to restore the Symmetric Key, if at all you have taken any backup. If not, only way is to delete the symmetric key and then restart the reporting services to resolve the issue.

You can follow below link to delete the Symmetric Key

http://www.mssqldestination.com/blog/manjunathcbhat/2013/reportserver-deleting-encryption-key-sql-server-reporting-services-configuration-manager