Dell Precision 7920 won't post but no errors

This workstation got stuck on Dell logo after a powercut

Reboot - stuck on logo screen

Hard reset - blank screen, no warning lights, one amber flash, then solid white led

BIOS reset and new battery - same

Tried another graphics card and monitor - same

Removed all memory and then just replace two modules at random - same

Reseated processors - same

Just put in one processor in wrong slot (intentionally) - warning led sequence

I am completely stumped!

When I turn it off it turns back on by itself, I assume because that's what was set in BIOS (can't remember)

In the meantime I have built a new workstation (had enough of Dell!) but I need to get some files off my local RAID 5 array and last back was not recent enough. So I just need it to boot once!

The lack of warning leds sequence and getting the solid white led and yet no video, is very odd

Feels like motheboard and yet why solid white led?

Any suggestions?