Recover GRUB after windos installation

First, open the Terminal.

Mount the partition you 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. Example: sudo mount /dev/sda5 /mnt. Then bind the directories, you need, like so:

sudo mount /dev/sdXY /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc

Now we jump into that using chroot.

sudo chroot /mnt

Now install, check, and update grub. This time you only need to add the partition letter (usually a) to replace X. Example: grub-install /dev/sda, grub-install –recheck /dev/sda

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

Now you can exit you mounted hard disk, and unmount.

exit
sudo umount /mnt/dev
sudo umount /mnt/proc
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.