OK, so I opened up my system case and futzed around a bit with the PSU switch off. Then I turned that switch on, and the system would not boot. The lighting was on on the ASUS x509 ROG board and the AMD cooler but the fans would not spin. And the board did not go through the bootup process. What the hell just happened, I asked myself.
THEN I REMEMBERED I HAD TO PRESS THE POWER BUTTON on the system case. Sure enough, it booted up just fine.
I've been building systems for over 30 years now, going back to 486 CPUs. How could I make such a colossal noob, double-noob mistake?