I remember a live CD/DVD where I could just boot into my GNU/Linux partition in case your grub was shot. The simplest thing was once in your GNU/Linux partition, install grub-pc and its brethen and simply do update-grub and all would be well. I tried grml and know it's not grml, maybe knoppix or some other live-rescue distro ?
the simplest way is to use isohybrid_bios_efi_grubrescue image meaning you can boot either as cd or usb on either oldPal_biosPC or on newer uefiPC and you are in grub commands in any of these cases see demo and explanation here
tested on "old i686 2GB" and on "Acer Nitro efi 4Gb"