Definitely not something you want to see on your second day back at work:
The report server cannot decrypt the symmetric key used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content.
Luckily my customer had a recent backup of the SSRS key and it was a simple matter to restore the key and restore SCOM reporting functionality.
You may be asking “How do I backup my SSRS encryption key?” no worries it is a simple procedure, I recommend making this process part of a monthly set of tasks.
First open your SQL Reporting Services Configuration Manager
Then navigate to Encryption Keys on the left hand side
Then click Backup, you will be promoted for a file name and a password
Restoring the key is similar you just select Restore from the same page and locate the key file and provide the password.