How to Repair, Restore, or Reinstall Grub 2 with a Ubuntu Live CD or USB

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　　Grub 2 typically gets overridden when you install Windows or another Operating System. To make Ubuntu control the boot process, you need Reinstall (Repair/Restore) Grub using a Ubuntu Live CD.
Warning
　　Using the sudo command, especiallyfrom a Live CD can do serious damage to your system. Read all instructions and confirm you understand before executing any commands. When pasting into the Terminal, use Ctrl+Shift+V, NOT Ctrl+V.
　　sudo mount /dev/sdXY /mnt
Terminal Commands
　　Mount the partition your Ubuntu Installation is on. If you are not sure which it is, launch GParted(included in the Live CD) and find out. It is usually a EXT4 Partition.Replace the XY with the drive letter, and partition number, for example: sudo mount /dev/sda1 /mnt.
　　sudo mount /dev/sdXY /mnt
　　Now bind the directories that grub needs access to to detect other operating systems, like so.
　　sudo mount --bind /dev /mnt/dev
　　sudo mount --bind /dev/pts /mnt/dev/pts
　　sudo mount --bind /proc /mnt/proc
　　sudo mount --bind /sys /mnt/sys
　　Now we jump into that using chroot.
　　sudo chroot /mnt
　　Now install, check, and update grub.
　　This time you only need to add the drive letter (usually a) to replace X, for example:
　　grub-install /dev/sda,
　　grub-install –recheck /dev/sda.
　　update-grub
　　grub-install /dev/sdX
　　grub-install --recheck /dev/sdX
　　
　　
　　Now grub is back, all that is left is to exit the chrooted system and unmount everything.
　　exit &&sudo umount /mnt/sys &&sudo umount /mnt/proc &&sudo umount /mnt/dev/pts &&sudo umount /mnt/dev &&sudo umount /mnt
　　Shut down and turn your computer back on, and you will be met with the default Grub2 screen.
　　You may want to update grub or re-install burg however you like it.
Congratulations, you have just Repaired/Restored/Reinstalled Grub 2 with a Ubuntu Live CD!