for whatever it’s worth, my powercolor red devil 5700 died in a similar way to this several years ago. Got to a point where it failed to output any display signal at all.
That’s what I thought too but as I’ve mentioned, keyboard and mouse are not getting any power from motherboard.
If it was GPU issue, those would have active lights with only monitor not getting any signal from GPU.
Are you getting WHEA logs here? Do they implicate a specific component?
When the system still used to turn on properly, I’d get these in Windows’ event viewer:
Reported by component: Processor Core
Error Source: Machine Check Exception
Error Type: Cache Hierarchy Error
Processor APIC ID: This kept changing
Provider Name: Microsoft-Windows-WHEA-Logger
Event ID: 18
When I started looking up information about this error I found out it could be caused by literally anything. Faulty CPU, board, memory, PSU. Sometimes setting CPU voltage around 1.30-1.35 helped. In one case the guy replaced his custom power cables with default ones and that solved the issue.