First, boot from a Ubuntu Live CD or USB
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 that you've done that, you need to bind the directories that grub needs access to detect other operating systems using the following commands.
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 you need to jump into it 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 --recheck /dev/sdX
Now that grub is 'back home' on your computer, you need to exit the chrooted system and unmount the drives
sudo umount /mnt/sys &&
sudo umount /mnt/proc &&
sudo umount /mnt/dev/pts &&
sudo umount /mnt/dev &&
sudo umount /mnt
Just restart your computer and you'll be greeted with the default GRUB selection screen. Select Ubuntu and you're done!