I have two, one is actually complicated and one was so obtuse that I never would have figured it out in a million years:
Actually complicated: I still don't know how it happened, but somehow an update on Arch filled the boot partition with junk files, which then caused the kernel update to fail because of no disk space, which then kind of tanked the whole system. It took ages, but with a boot disk and chroot-ing back into the boot partition I eventually managed to untangle it all. I was determined to see it through and not reinstall.
Ridiculous: One day when using Ubuntu, the entire system went upside-down. As in, everything was working perfectly fine, but literally the screen was upside-down. After much Googling I had no luck figuring it out, then I accidentally found the solution - I'd plugged a PS4 controller into the USB on the laptop to charge it, and for some reason Ubuntu interpreted the gyroscope on the controller as "rotate the screen display" so when I moved it, the screen spun round. I only figured it out by accident when I plugged it back it and it spun back to normal lol.
Ridiculous: One day when using Ubuntu, the entire system went upside-down. As in, everything was working perfectly fine, but literally the screen was upside-down. After much Googling I had no luck figuring it out, then I accidentally found the solution - I’d plugged a PS4 controller into the USB on the laptop to charge it, and for some reason Ubuntu interpreted the gyroscope on the controller as “rotate the screen display” so when I moved it, the screen spun round. I only figured it out by accident when I plugged it back it and it spun back to normal lol.
I had a similar one. I had a usb-powered fan cooling pad that my laptop was sitting on. My laptop would randomly go into boot loops when I turn it on. I thought it was a grub issue so I always had my usb stick ready to re-install grub. Did some dusting one day and forgot to plug in the cooling fan, then the boot loop never happened again. Turns out it was the fan plugged into the usb that was causing it.
Semi-related note.. displayport cables can cause a no-boot condition too. I think it was the existence of Pin#1. I had to duct tape that one pin and my computer finally booted up.
A couple years ago on Reddit I saw a story where a dude working IT support had to drive to a remote office or replace a workstation that wouldn't boot. When he got there the lady whose desk it was had some shitty USB fan or maybe an led Christmas tree plugged into one of the USB ports. He unplugged that and the pc booted fine.
Ah I remember that one! Classic. I also remember a story about someone who lost an entire PC in their apartment. It was running and connected to the network, they could ping it, but couldn't physically find it lol.