If you start your Ubuntu and it display a grub shell to you, you can run this command below to fix it:
grub-rescue> prompt
ls
set prefix=(hdX,Y)/boot/grub
set root=(hdX,Y)
set
ls /boot
insmod /boot/grub/linux.mod
linux /vmlinuz root=/dev/sdXY ro
initrd /initrd.img
boot
* Ensure that you entered correct partition (e.g. hd0,1) for hdX,Y. If not, than your grub will be messed.
References :
http://superuser.com/questions/181733/how-can-i-restore-grub-without-a-live-cd1
http://www.linux.com/learn/tutorials/776643-how-to-rescue-a-non-booting-grub-2-on-linux