How to Restore Grub 2 As The Main Bootloader

For those who are having a dual (triple or quad) boot system in your computer, chances are, your bootloader might break when you update one OS to a newer version. If you are using Ubuntu (with the new Grub 2 bootloader) and you installed Windows. The Windows bootloader will erase your Grub 2 and you won’t be able to boot in your Ubuntu. As such, you will need to restore your bootloader to Grub 2.
Note: If you are still using the Grub legacy, check out this post to restore your Grub.
Requirement: Ubuntu (Karmic or Lucid) LiveCD (download it from Ubuntu.com)

Start your computer and boot into your Ubuntu LiveCD.
The first thing that we need to do is to mount your existing Ubuntu partition. If you already know your partition number (of the form sda1, sdb2 etc), you can easily mount using the following command:

sudo mount /dev/sda1 /mnt    #replace sda1 with your partition number

If however, you have completely no idea of your partition number, launch GParted (System -> Administration -> gParted). Locate your Ubuntu partition and record down the partition number.



If you are are using different partitions for the root and the home folder, make sure you record the root partition instead of the home.
Close the GParted. Open a terminal and type in the following command:

sudo mount /dev/sda1 /mnt    #replace sda1 with your root partition number

Next, all you need to do is to reinstall Grub 2 with the following command:

sudo grub-install --root-directory=/mnt/ /dev/sdX  
#replace the X in sdX with your partition alphabet

Reboot.
You should be able to boot into your Ubuntu now.
Run the following to update your Grub 2 to include the bootloader of Windows (or other OS).

sudo update-grub

You should be able to boot into your Ubuntu and Windows (or other OS) now. Source: http://maketecheasier.com