A). Commands in the video
Prerequisites:
            1. A Linux install media (any GRUB-based distribution with a live ISO should do)
            THAT'S IT!

Commands to reinstall GRUB (from a live ISO):
            List devices and partitions: sudo fdisk -l
            Mount root partition: sudo mount /dev/sdXY /mnt (example: sudo mount /dev/sda3 /mnt)
            Mount boot partition (if applicable): sudo mount /dev/sdXY /mnt/boot (example: sudo mount /dev/sda2 /mnt/boot)
            Mount EFI System partition (if applicable): sudo mount /dev/sdXY /mnt/boot/efi (example: sudo mount /dev/sda1 /mnt/boot/efi)
            Reinstall GRUB: sudo grub-install --root-directory=/mnt /dev/sdX (example: sudo grub-install --root-directory=/mnt /dev/sda)

Commands to boot from GRUB console:
            List devices & partitions: ls
            Set root partition: set root=[partition] (example: set root=(hd0,gpt3))
            Specify vmlinuz file and root partition: linux [vmlinuz file] root=/dev/sdXY (example: linux /boot/vmlinuz root=/dev/sda3)
            Specify initrd image: initrd [initrd image] (example: initrd /boot/initrd.img)
            Boot the system: boot
*Don't forget to run "sudo update-grub" from within your Linux installation after doing this!*

Other commands used in this video:
            List partition UUIDs: lsblk -o uuid
            Edit fstab file (from a live ISO): sudo nano /mnt/etc/fstab

For anyone with a separate /boot partition, when specifying your "root" at the GRUB prompt:
            set root=[*BOOT* partition] (example: set root=(hd0,gpt2))
            linux [vmlinuz file] root=[*ROOT* partition] (example: linux /vmlinuz root=/dev/sda3)
Your vmlinuz file and initrd image are at the root of your boot partition.

B). Commands from a different video

Repairing from a GRUB > Prompt
Find your Hard drive
ls
Output Example:
(hd0) (hd0,msdos2) (hd0,msdos1)

List Root partition on each drive til you find your Install
ls (hd0,msdos2)/
Check for the directory listing, if nothing, move to next

Now that we have found the proper drive lets boot to it
grub> set root=(hd0,msdos2)
grub> linux /boot/vmlinu (tab complete) root=/dev/sda1 (or the root linux partition)
grub> initrd /boot/initrd (tab complete)
grub> boot

Repairing from a GRUB Rescue > Prompt
Find your Hard drive
ls
Output Example:
(hd0) (hd0,msdos2) (hd0,msdos1)

List Root partition on each drive til you find your Install
ls (hd0,msdos2)/
Check for the directory listing, if nothing, move to next

Now that we have found the proper drive lets boot to it

grub rescue> set prefix=(hd0,msdos2)/boot/grub
grub rescue> set root=(hd0,msdos2)
grub rescue> insmod normal
grub rescue> normal
grub rescue> insmod linux
grub rescue> linux /boot/vmlinu (tab complete) root=/dev/sda1 (or the root linux partition)
grub rescue> initrd /boot/initrd (tab complete)
grub rescue> boot

grub

Fix GRUB Permanently
Now that we are booted in from GRUB Rescue, we can begin work with repairing our grub permanently. First we rebuild the /boot/grub/grub.cfg file:

Debian-based Distributions use update-grub
Other Distributions use grub-mkconfig -o /boot/grub/grub.cfg

With the Configuration rebuilt, we now simply need to reinstall grub

grub-install /dev/sda

Note: /dev/sda is the DEVICE…NOT THE PARTITION.

Other Methods of Repairing GRUB
Use the [https://www.supergrubdisk.org/ image on a USB drive to repair][1]
Use a vanilla server distribution pen drive to boot to prompt, then chroot to your existing install and reinstall GRUB.