A grub breaking thingy happened to me too.
I was saved by having multiboot, with every OS having their own GRUB version installed. (just selected one using the motherboard's interface)
The problem occurred when, after pacman -Syu, I read notes in the output, one of which hinted I would want to update GRUB and went - "Sure, I'll try the new GRUB update" and ran GRUB update.
When it didn't startup after a restart, I just used the debian's GRUB to login to the OS in question, downgraded GRUB, reinstalled GRUB and then ran pacman -Syu again.
I feel like mine wasn't the problem instance that goes on around the web, mostly because:
None of the mentioned fixes worked in my case.
I feel like people won't go out of their way to update GRUB most of the time.
