Request root permission:
Determine your normal system partition. The following commands may be helpful. The fdisk switch is a lowercase "L".
fdisk -l blkid df -Th
Mount your normal system partition. X is the drive letter. Y is the partition number:
Substitute the correct partition: sda1, sdb5, etc. sudo mount /dev/sdXY /mnt
mount /dev/sda1 /mnt
Only if you have a separate boot partition (where sdXY is the /boot partition designation):
mount /dev/sda3 /mnt/boot
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:
Reinstall GRUB 2 (substitute the correct device with sda, sdb, etc. Do not specify a partition number):
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):
Exit chroot: CTRL-D on keyboard, and reboot.