Wow, new post and techs already gotta argue over who's smarter, typical.
Look, by the way the OP asked for help, he identified himself as a neophyte, and that's perfectly great - so let's start at step 1. Yes, an experienced person would be able to skip steps, not so the neophyte.
As it was on and suddenly off with no response, yes, we look at power - breaker, PSU, PSU switch, power cord. Could also be the cable from PC switch to MB, but system was on and even if you disconnect that switch while PC is on, it doesn't turn the system off - so, not likely. Also not likely the power cord cause rare failure, but it DOES occasionally happen.
Unplug PC and turn off PSU switch - wait a minute or so - plug back in, turn PSU switch on. Verify power cord is fully seated in PSU. Try to turn on
Verify wall power - reset the breaker if equipped (seen do partially trip at times). Verify power by turning on PC or plug in another device. Can even take PC to another known working outlet to be sure.
Still nothing? Now we're down to power cord (unlikely), PSU, and PC switch / cable to MB (also unlikely).
Power cords "do" fail - it's RARE, but.....if you have another can try swapping it out.
The switch connector on the MB operates as a momentary short - I really don't think it's the issue, but can always short the MB pins (only if comfortable doing so).
If still nothing, now we are down to the PSU as failure.
For myself, I've had multiple times when I'd hit the switch and nothing would happen - and it's NEVER been the PSU. Hell, my PSU survived lightning coming into the house through cabling (house full of smoke, lots of stuff dead including a hole in my A/C coil in the attic - and yes, I have "funny" power issues on occasion now).
And yeah, it's possibly the PSU...but maybe not. Troubleshoot before part swapping (when it isn't obvious)
And to anyone here trying to help folks - seriously, thanks