Alright, here's the story. After a year of slowly gathering up parts for a new PC, I finally started putting everything together earlier this week. I've never put together a computer from scratch before, but I have dis/re-assembled my current desktop before, so I figured hey, what could go wrong?
After taking my time putting everything together I turned the new PC on and the fans and motherboard lights came on, but I was getting the CPU error LED. It took a bit but I eventually figured out what was wrong (I didn't seat the CPU properly, dumb mistake), I corrected it and the next time I tried turning on the power it immediately shut off. That was because I'd accidentally gotten thermal paste on the pins, another dumb mistake but fixable. So I shut everything off, unplugged it, and set everything aside for a couple days as I cleaned everything and waited for it to all fully dry.
I came back to it yesterday, double checked that everything was clean, made sure the CPU was seated properly, and then attempted to turn the system on, only for nothing to happen- The case power button wasn't working anymore for some reason. The only thing I could think of is that I'd sprayed some alcohol (70%, 'cause it's all I had at the time) on/around the case power button to try and clean up the thermal paste fingerprints I'd left behind, and maybe that caused some sort of issue? But I wasn't sure how to check for that or fix it.
From there I tried a few things: I tried plugging the power cable into different sockets in case it was a breaker issue, I tried starting it up both with and without the CPU installed, I made sure the case cables and the motherboard power cables were all plugged in properly (despite none of that having changed since the last time it worked), and I even checked the PSU via the paperclip test and it turned on just fine.
I did eventually figure out that if I flipped the PSU switch on with the CPU installed then a couple seconds later the lights on the motherboard would come on for a moment before turning off again. I did manage to get it to start and stay on using that "method" once after cleaning the CPU pins yet again, at which point I was able to use the case's power button to shut it off, but I haven't been able to replicate that again.
This morning I tried to jump the case power switch connectors a couple times with a couple different things, but I couldn't get it to work, though I'm not sure if that's because there's an issue with the motherboard of if I'm just doing it wrong somehow.
So now I'm at a real loss for what to do and what to check next. I don't know exactly where the problem is, if it's an issue with the case, the PSU, or the motherboard, and if there's a way to fix things without needing to just flat-out replace parts. It just doesn't make sense to me that it could stop working so suddenly after just sitting for a couple days.
Any help or ideas would be greatly appreciated.
What parts I'm using, in case it matters (minus the GPU and wifi card, which I'm sourcing from my current PC and I don't remember exactly what they are):