First, boot the affected computer from a live usb or cd. Then mount the root file system on /mnt (assumes the hard drive you want to install grub on is /dev/sda).
mount /dev/sda2 /mnt
If necessary, mount /boot
mount /dev/sda1 /mnt/boot
Mount the virtual file systems /dev/, proc, etc
mount --bind /dev /mnt/dev
mount --bind /dev/pts /mnt/dev/pts
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
Chroot into the mounted environment.
sudo chroot /mnt
Install grub.
grub-install /dev/sda
Generate a grub config file.
update-grub
Exit the chroot environment and unmount the file systems.
exit
umount /mnt/sys
umount /mnt/proc
umount /mnt/dev/pts
umount /mnt/dev
umount /mnt
Reboot!
No questions yet.