Boot into the Ubuntu Live Cd and open terminal.
1. Identify Ubuntu partition:
sudo fdisk -l
2. Mount it ( replace sda1 with the Ubuntu partition number):
sudo mount /dev/sda1 /mnt
3.Reinstall grub:
sudo grub-install --root-directory=/mnt /dev/sda sudo update-grub
Solution from: grub2 – Ubuntu 11.10 not showing up in GRUB bootloader after update and restart 😦 – Ask Ubuntu.