Question PC doesn't turn on after experiencing some bizarre issues beforehand ?

[schwwwk]

My PC doesn't turn on at all - nothing happens when I press the power button. Fans don't turn on etc. My GPU and USB devices usually light up when connected to power (even if the computer is off - the PSU just needs to be on), and they still do this. This is the only sign of life I can see.

Background
Before it broke I had a GPU update cause issues (this is not unusual). I spent some time trying and failing to fix this. I decided to use Timeshift to restore my system files to the state they were in pre-update. After this I witnessed a chain of increasingly more bizarre software issues until my PC failed completely:

• Timeshift reboots after it finishes restoring files. After this reboot I was no longer able to boot into my OS: I could access the grub menu, but after selecting my OS I was stuck on a blank screen. No errors. This was strange to me, because I've used this software for years and it always worked as expected.
• My BIOS settings menu allows mouse input: I can move and click my mouse. This time, I could move my mouse, but clicks did not register.
• I booted into a live USB to look at my boot drive. I could move my mouse but right clicking and left clicking still did not work. Looking at my storage drive, I noticed that many folders such as /boot were empty when they shouldn't be - so no wonder I couldn't boot in! Interestingly enough, the drive reported that I had the same amount of storage free as before I started the restore, even though a lot of my system files were missing.
• I turned my PC off. When I pressed the power button to turn it back on, nothing happened.

I tried plugging the "broken" mouse into another computer, and it worked fine.

Things I have tried

• Removing storage devices and one stick of RAM, leaving it with just the essential components needed
• Replacing my GPU with a lower-powered one
• Resetting the CMOS battery

No change after doing those things.

What now?
I have a hunch it's an issue with the motherboard. My mouse not working on this PC but working on other computers would indicate there's a problem receiving data from the USB ports, right? So I am guessing I will need to replace that. But I don't know for sure if it is the motherboard, if it's other components, or both.

I'm considering buying a new PSU regardless of whether or not it's broken, as I don't feel great about my current one. It's alright but there is no undervolt protection. If it is broken, I don't want to risk buying new components and having it immediately fry those.

I'd appreciate any thoughts on the matter. I don't have easy access to other components, so I'm not sure where to go from here besides blindly buying new parts.

Specs
MBD: Gigabyte B450M S2H
CPU: Ryzen 5 1400
GPU: Asus GTX 970
RAM: Corsair Vengeance LPX 16 GB
SSD: Crucial P2 1TB M.2-2280
PSU: Silverstone ET550-G

Nothing is overclocked.

 

[Lutfij]

I'm considering buying a new PSU regardless of whether or not it's broken, as I don't feel great about my current one. It's alright but there is no undervolt protection. If it is broken, I don't want to risk buying new components and having it immediately fry those.
You should source(borrow, not buy) a reliably built 550W PSU and see if that alleviates the issue.

 

[schwwwk]

I'm considering buying a new PSU regardless of whether or not it's broken, as I don't feel great about my current one. It's alright but there is no undervolt protection. If it is broken, I don't want to risk buying new components and having it immediately fry those.
You should source(borrow, not buy) a reliably built 550W PSU and see if that alleviates the issue.

That would be optimal for sure, unfortunately I don't know where I would be able to borrow one from.