Substitute the correct partition: sda1, sdb5, etc.
sudo mount /dev/sdXY /mnt
Example 1: sudo mount /dev/sda1 /mnt
Example 2: sudo mount /dev/md1 /mnt
Only if you have a separate boot partition (where sdXY is the /boot partition designation):
sudo mount /dev/sdXY /mnt/boot
Example 1: sudo mount /dev/sdb6 /mnt/boot
Example 2: sudo mount /dev/md0 /mnt/boot
Only if (some) of the system partitions are on a software RAID (otherwise skip this step): make sure the output of mdadm –examine –scan agrees with the array definitions in /etc/mdadm/mdadm.conf.
Mount the critical virtual filesystems. Run the following as a single command:
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
Chroot into your normal system device:
sudo chroot /mnt
Reinstall GRUB 2 (substitute the correct device with sda, sdb, etc. Do not specify a partition number):
grub-install /dev/sdX
If the system partitions are on a software RAID install GRUB 2 on all disks in the RAID. Example (software RAID using /dev/sda and /dev/sdb):
grub-install /dev/sda
grub-install /dev/sdb
Recreate the GRUB 2 menu file (grub.cfg)
update-grub
Exit chroot: CTRL-D on keyboard
Reboot.
sudo reboot