How to restore Grub in Ubuntu

Boot up your live CD
In the desktop, open terminal (Applications -> Accessories -> Terminal).

sudo grub

This will set it to grub mode

find /boot/grub/stage1

This will locate your boot partition. If you already know the location, you can ignore this step.

root (hd0,0)

Replace the (hd0,0) by your boot partition number. If your Ubuntu is installed in the second partition, then change it to (hd0,1)

setup (hd0)
quit

Reboot your system. You should be able to access the Grub bootloader now.