Earlier my PC (which has had on-and-off bouts of instability almost since it was built, leading some on this forum to suspect corrupted OS files) had a repeated failure to boot, with the motherboard's "easy debug" light showing a CPU failure every time, but the drives and fans visibly and audibly working. Removed and reseated the CPU as a desperation shot, new thermal paste, snugged up all the power cables at the PSU end, just sort of hoping something would do something. Plugged it back in, accidentally hit the power button as I was doing so, no CPU failure light, but a VGA failure light as I didn't have the monitor plugged in yet (I'd just got it down there and started trying). Plugged it in, nothing, now there's a BOOT light showing. Turn it off, finish plugging everything in, turn it back on, boots fine with no failure light, disk check and I'm good to come post this right now. Apparently I'm far from the only person to have "boot failure with CPU light" happen on a Ryzen; I've seen the problem traced to both the CPU and the motherboard on other people's builds, along with a very long list of other problems that would basically necessitate disassembling and reassembling the entire system, then ruling out about half its parts (which I don't have enough spares to do). Could even be an entirely separate issue from the other instability!