|To me, this appears like a PSU failure... it's quite a possibility. A single bad capacitor can completely kill the PSU.|
But to start from the top, as this appears to be a custom build: Did you check all the plugs on the rails feeding the motherboard? Having this wire(s) come loose can completely shut down your system as described, AFAIK. Many fans feed from the motherboard and no power, no fans, no system period. Light on the GPU showing power probably means that at least some of the supply wires are still working, but I still wouldn't rule out complete PSU failure. It does happen, even with the best brands.
Not everyone can decipher Klingon script...
chay' ta' SoH tlhe' vam Doch Daq