Mount the linux partition, chroot to it, run grub-update.

Or:

  • fdisk -l list of physical devices (X) and partitions (Y) (or blkid)
  • mount /dev/sdXY /mnt
  • umount /mnt/boot and mount /dev/sdXY /mnt/boot if you have a separate boot partition
  • grub-install --boot-directory=/mnt/boot /dev/sdX

(Given you have access to a busybox/linux shell)