So here's an interesting problem.
I was about to turn off my computer about 2 weeks ago when I noticed something strange in Task Manager. I have the CPU graph set to see the logical processors individually, and I saw that the usage for core 0 specifically was virtually non-existent. I booted up a game and saw the same thing (image for reference: https://imgur.com/8S2kVVK). I double checked in HWMonitor and it was the same thing. I rebooted my PC and the issue did not persist. Didn't think much of it at the time.
Fast forward to about a week ago and I noticed the same thing again. I tried to force CPU core usage to all cores in the advanced boot options of MSCONFIG and it was fine after another reboot. The issue seems to begin after the computer has been on for a while (approx. 3-5 hours or more), but I have not caught the exact moment the core usage drops to zero.
I read online about individual CPU cores failing, but I have received mixed information on whether a single core failure would make the CPU unusable. Even more interesting is that the hyperthreaded core (core 1) is still working, so this has genuinely stumped me. I have not noticed a significant performance hit while gaming or anything, though I think overall usage on the CPU has gone up slightly while playing the same games on the same settings, and no obvious problems have shown themselves since I first noticed this happening.
Can anyone shed some light on what this could be? Is this likely a hardware issue (I don't see how with the virtual core still working fine) or simply windows reporting the usage incorrectly?