Raidmax is very nearly the worst PSU brand in existence, so it wouldn't surprise me if the PSU died in under 4 hours.
I wouldn't... expect... that even for Raidmax, but I also wouldn't put it past them either.
However, it is possibly a little more complicated than that.
The way you turned on the PSU without it plugged into anything, that was by letting power flow through the +5sb line which is the same thing that controls the power button and everything else that needs power when the system is off.
That line can only handle a very small amount of wattage, if that wattage is overloaded, then something like the power button or the keyboard that is necessary to wake up the computer from its power saving modes wouldn't get enough power to work right and it wouldn't be able to wake up.
If you had a lot of USB devices plugged in or something like that, this could be the problem, because USB devices suck power when the computer is off and they can redirect power away from the power button control circuits.
However, the problem could also be a general power settings problem if you are able to turn the computer off and back on with the power button.
If a computer has trouble waking up from sleep mode, its very often a bad PSU, but for new PSUs often it implies that there are problems with the way power save modes are configured. Like if devices get shut down in the wrong order then they can have trouble waking up again even if the PSU is good enough.
My wife has had that problem before in her PC. If she leaves it on and it does a Windows Update or something overnight sometimes she comes in in the morning and she can't wake it up without turning it off and back on because of some improper order of part shutdown or something.
- Edit 2 - I want to say this has to do with the monitor being in sleep mode when the computer restarts itself after an update, or something like that.
If you really just can't get it to come on at all, though, the PSU may have damaged one of the internal parts during that time you were gone.
As I mentioned before, Raidmax is on the worst brand shortlist, but that even kinda sounds like a stretch to me that it would break inside of 4 hours.
If you turned it on just fine and installed windows just fine and then you came back and it didn't work there would be no real value in breadboarding anything. Breadboarding would just rule out a short, but having it work right the first time would also rule out a short and that was already done.
It is OK that you don't have a way to test voltages, because voltage meters don't tell you anything you need to know about how PSUs function. There are power correction circuits inside the PSUs that can detect a voltage problem within milliseconds and fix it. That is faster than a voltage meter can determine there is a problem.
PSUs that are bad today are in spec, out of spec, in spec, out of spec, in spec, out of spec. To voltage meters, it looks like they are always just in spec. I have never heard of one person putting a voltage meter to a PSU and finding a problem with it.
It would be different if you were checking the circuit breaker for your house and you can see that its either 120v (working) or if somebody dug up the ground in the wrong place and cut the cable (0v), but that is about the extent of the usefulness of these things. Their applicability to computers is very little.
All you can really do right now is try to get access to computer parts similar to your own and try switching out yours for the other ones.
Welp was able to borrow a PSU tester. Plug up 20pin comes on no problem, connect one cpu lead good to go, connect the other cpu lead, blammo short and tester goes blank. RMA'd at least newegg has decent RMA policy. Just hope it didnt take the processor or mobo when it went=/