I have had a weird occurrence. About 3 weeks ago I had the MB moved from one case to
ANOTHER.  After words I was moving data from various HDD (which had OS on them so they were mounted as slave)  As a result I was doing frequent  reboots. I was doing a reboot when the POST got to the point of simply recognizing that there was a PRIMARY slave and just stopped. No matter what I did the system would not respond.. If  I REBOOTED the same thing happened. So I unplugged it and let it sit a couple of days until I got get it to the shop. When I got it to the shop they plugged it in and it worked perfectly.
A couple of weeks later I was again doing frequent reboot when it happened again, except this time POST only go to the point of checking “NVRAM” when it stopped.
I decided to let it sit unplugged for a day. When I booted it again this time it go to the point of recognizing the primary master and stopped. I let it sit unplugged for another couple of days. Booted it again this time it worked perfectly since then. ??
Replace the CMOS battery...part # CR2032 available anywhere.

You may want to start SAVING up for a new PSU...