Fell into the old trap!
The boot order of the Hard drives had changed itself round and it was trying to boot from the wrong drive! Not sure why that happens but it does.
Changed it back and it's running fine.

I had to reboot my server the other day and it now fails to reboot. It stops at a black screen with a blinking cursor at the top left and doesn't respond to anything except the power switch. I have 4.2Tb of storage in there which is greyholed and has been working fine for some months. I can boot the...