My computer had been on for a couple days and was working fine. I was following instructions for booting into the BIOS update utility, which said to completely shut the machine down, power and all, and then restart with the BIOS update USB stick inserted.  Well, the first part went fine and the machine powered down.  Now it appears to be dead.  When I press the power button nothing, and I mean NOTHING: no flickers, no fan twitches, no sounds, no lights no NOTHING.  However, there is a sort of "pilot light" (for want of a better term) on the graphics card (an ASUS GeForce GTX 1080) that *is* light when the power cable is plugged in to the wall ... so SOME power is getting through.  I just bought and installed this supply in mid January 2018, so it is barely 6 months old:
 
Part Number: 110-BQ-0500-K1
Part Desc: EVGA 500 BQ, 80+ BRONZE 500W, Semi Modular, FDB Fa
 
Is there any sort of reset switch or anything that might have tripped on the supply that I need to get into the case to reach?  Is there anything specific I should check to see if the PSU is still working at all and if it has failed, how could powering off cause a failure of a working PSU?

OK, I tracked down a PSU tester and the PSU is fine.  In fact, when I reconnected the PSU to the mobo, it was willing to try to boot -- ONCE.  Next time it went through a power cycle, same problem.  So it is NOT the PSU.  Sorry to be a bother.
 
 

I did not try removing the graphics card.  I did, however, discover that the problem vanishes completely if I replace my normal HDMI-connected display with a DVI-connected display.  In fact, simply disconnecting the HDMI cable from the graphics card would boot the computer!
Regardless, the problem is most definitely NOT the EVGA PSU.  I just had to ask my question after about four hours of banging my head with everything I could think of.  Then I let it rest for a couple hours and had new ideas that finally got me running again, at least.  I may need to reboot with the monitor disconnected for awhile, until I figure out a real solution, but that will be livable, if annoying.
But my 500 BQ is flawless.  In fact, I have TWO (in 2 different machines) and both run super quiet and smooth!
So this thread may be closed or even removed.