I don't know if it was a rogue update or something but I had the same (or similar) problem. Booting dropped me to the grub rescue prompt and my i386-pc folder was also empty.

Only strange part of my setup is the main drive is GPT partitioned with regular BIOS so I have a small bios_grub partition at the beginning of the drive for the stage 2 bootloader.

Somehow my /boot filesystem got corrupted and after a fsck and copying the files from /usr/lib/grub2/i386-pc to /boot/grub2/i386-pc I was able to boot again.