Comment 7 for bug 30123

A lot of people are continuously looking for help with this matter around the internet. Too often the instructions they get are partial, false, or not working for some other reason. It's not rare to see people to be told to reinstall Ubuntu, because it's easier that way! Take a look at here what it's like:
http://ubuntuforums.org/archive/index.php/t-24113.html

This is not a minor problem. People face this always when they reinstall Windows without reinstalling Ubuntu at the same time (and Windows typically has to be reinstalled every now and then). A one click (or otherwise simple) option for reinstalling grub is definitely needed. Please please code wizards, give us this feature!

I marked bug #72213 as a duplicate of this.