Several Ubuntu installations, reinstall your GRUB after an upgrade

Maybe your HD is like this:
Ubuntu 14.04 on /dev/sda1 Xubuntu 14.04 on /dev/sda5

If you upgraded one of them, some times the GRUB changes of partition. How to fix? Easy, startup from the desire partition and exec:
sudo grub-install --root-directory=/boot /dev/sda
update-grub

Source

If you run from a LiveUSB:
sudo mount /dev/sda1 /mnt 
sudo grub-install --boot-directory=/mnt/boot /dev/sda