Power Blackout Problem

T'were it mine I'd unplug the power cord, let it set a few minutes to drain any residual power from the motherboard, open the case and remove the CMOS battery or if there's a jumper or switch use it to reset the BIOS, should be mentioned in the board's manual. While in there might also remove and replace [reseat] the memory modules and any add-in PCI or PCIe card. I have an ASUS motherboard as an OEM but it came with an Add-in PCIe X16 video card, no onboard adapter. Might also unplug the power supply to motherboard cables, just remember where they go. Having built it yourself shouldn't be an issue in essentially redoing things.

If you can then get into the BIOS I'd choose the Default Options to at least see if it will boot, can make custom changes later.

Generally, after a power outage, you just press F1 to enter Setup (the BIOS), then you hit F10 to save and exit.
That fixes things 99% of the time.

I do have a bootable USB thumb drive for emergencies, it is over two years old though, and I'm not sure if it is still relevant.

Once you've dealt with BIOS/UEFI issues as above- if you still can't boot... you could consider using that just to see if your PC will boot to something, of course, and operate stably.
You don't say what sort of bootable drive it is.

E.g. If it's a Win 10 boot disk, then via 'Repair your computer', continue to Advanced Startup options and e.g. to a command prompt. I.e. don't use it to install anything!

If it gets to that, it's likely that a significant part of your hardware is ok- but says nothing about the state of your system disk of course.

If it's a live boot disk you could boot your system from that, and if that succeeds, you could check your internal disk, view files on it etc.

I will assume you haven't yet discovered the value of using disk imaging routinely and regularly so you can restore imaged partitions/your O/S to a previous good state, giving you a second chance if things go badly wrong - e.g. if your O/S is unbootable or your system disk has failed.

Macrium Reflect (free) + large enough external storage for image file sets.

Gudzilla said:

Thanks for all the ideas. I don't know if it was the residual power or not. So, I just switched keyboards and pressed F1 when I started it and I got into the BIOS, and I'm back in business. :)

Hope you're using disk imaging as I mentioned- especially if you have this problem.

Do remember to do a routine check of disks and file system after such an event.

Consider surge protection etc.

That happened to me once and I had to reflash the BIOS