The keyboard light only tells you that one of the PSU rails came on. It tells you little to nothing.
For THE DEAD PC the process is the same. You make the PC as small as possible then make it smaller till you find it or you are left with as few parts that don't power up.
For example I can put the motherboard and nothing else in the old motherboard shipping box, connect the PSU and try a power up test. This varies with motherboards but works 99% of the time as I rarely see a power up issue with just two items. Also no to "Don't you know what it is on my PC?"
Again, you make it smaller. That said it sounds like the old issue of too many motherboard standoffs where it finally scrapes through the motherboard paint and shorts out. I'd start as small as possible and build up testing as you go.
PC Suddenly Won’t Boot
I recently built my first pc around 2 days ago and everything has been working fine. So today I decided I want to route the HD audio cord through a different cut out so I shut down the pc, flipped the psu switch off, and went on disconnecting then reconnecting the audio cord. So I went to flip the psu switch back on and when I did a light on the mobo flashed and now my pc won’t boot. I know it has power cause my keyboard lights up, but when I hit the power switch on the front nothing happens. No fans or anything. I’ve reseated the front panel connectors and still nothing. Any help is appreciated as I am still a beginner and worried I may have broken something.