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.
grub-install /dev/sdX grub-install --recheck /dev/sdX update-grub
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.