There was no /etc/default/grub file, so how come my system was able to boot?
Reinstall the package and hope for the best:
sudo apt-get --reinstall install grub-pc
Actually the file is not critical for the boot, just to configure the boot when you execute sudo update-grub. If the file doesn't exist, then all values are default ones, like you didn't set them up.
just copy example from /usr/share/grub/default/grub:
sudo cp /usr/share/grub/default/grub /etc/default/