I will suspect of Power supply.
On Electrical cabinet you have it on left side.
Control for PS comes from a slim relay board mounted on left door
Same cabinet.(bottom of memory boards)
Perhaps one of them could be faulty.
As you describe failure, all power is shut down suddenly as blackout.
You can get somebody to power on the machine while you observe sequence of power at relay board mention and isolate which one actuates to power on Power supply.
Then you can swap that relay.
I wish you success.!