There are times when you inadvertently overwrite your Master Boot Record. The end result being that you are unable to boot into Linux. This is especially true when you are dual booting between windows and Linux OSes or installing/upgrading your windows OS
This is what you do to restore the GRUB2 boot loader when faced with the above problem. First you need a Linux distribution CD, Ubuntu Jaunty, Karmic Live CD.
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source:http://linuxpoison.blogspot.com/2010/10/13578167752244.html