Sounds like hardware. It is ether power supply, motherboard or cpu. Here are some tips to check. Pull out your multi-meter, set it to AC, check outlet, then check the end of the power cord that plugs into the power supply, how switch to DC and pull out your power supply manual and start checking pin voltages. If all of those are correct And stable, check the cpu socket. Any of the pins have corrosion or look melted? Check your motherboard over any melted spots, blown caps or corrosion? I believe your monitor should show your bios info even without functioning ram, and if the GPU was bad, you would hear the hdd spin up and the windows loading sound and just not see it. Oh and if the HDD was bad you would see the computer attempt to post.
Disclaimer. if you do not know how to use a multi-meter, learn the correct procedures first. Also becareful with electricity. If not handled correctly you will make matters worse or hurt/kill yourself. Lastly if you comfortable with electronics, I recommend seeking out a profession to isolate and repair the issue.