If you click on the Ticket link in the upper right of the web interface does it give you any RAS alerts. There would probably be a RAS ticket in there that will tell us why it is in Diagnostic mode.
In the web interface you can go to Utilities > Reboot & Shutdown then click on the Reset from diagnotic state to see if this error will clear. This will force a reboot of the system.
We would have needed to get more information from the system in order to figure out what was keeping the system in Diagnostic mode. Opening a Service Request would possibly have been needed.