I am so happy because many of my friends are migrating to Ubuntu now a days.There are so many links over net to re-install grub.Any way I am just listing the steps to do it.
1) Boot from livecd.
2) $sudo fdisk -l
This displays your partition table
3) $sudo mkdir /media/root
4) $sudo mount /dev/sdaX /media/root
sdaX is your linux partition,replace X with number corresponds to it which u can obtain from partition table
5) $sudo mount /dev/sdaX /media/root/boot
6) $sudo grub-install –root-directory=/media/root /dev/sda
You should get the output “Installation Complete,No errors reported”
7) Reboot and update grub.

This is the shortcut method.Please visit https://help.ubuntu.com/community/Grub2 , http://www.dedoimedo.com/computers/grub-2.html to know about Grub2 and re-installation.