Lots of PSU's have an internal fuse, sometimes soldered to the board. Check inside.

Usually when there's a power spike, it's the MOV that goes first, which then takes the fuse along with it. Check both.

That's assuming the PSU has any internal MOV, which many, specially veteran and/or cheap ones, don't.

It's a good idea to use a decent surge protector upstream of any comp.

Ok, now I'm just completely stumped. Not ONE of my 4 retro machines is turning on now. Not even the one I JUST built yesterday as the rescue machine.

I plug them in, I press the power button...nothing. 2 of these machines had nothing changed in them. Nothing. I tried different power outlets, different power cords, nothing seems to be a common point of failure. Can all of my parts have died at one point? Has an angry modern console gamer snuck into my man cave and specifically EMPd just my retro rigs?

This is extremely frustration here.

Disconnect the PSU completely and see if it powers up on it's own. Check if the fan starts spinning and if the voltage on the power rails is normal. For ATX you can short pin 14 (green wire) to GND (any black wire) to switch it on.

Seems as if I figured it out. I took a power cable and thr machines and plugged them into an outlet in a completely different room. All 4 turned right on. So I reset the circuit breaker at the panel for my computer room and that seems to have done it. The only actual hardware issue I had was I appear to have boughy a bad pc66 16mb ram stick for my dos time machine. Going back to the known working 32mb stick I had in there previously takes care of that.

I guess I freak out a bit too much when I post issues like this but as frustrating as this hobby can be with all the old hardware, I really enjoy it when I finally figure things out and everything just works as it should.