I’ve recently installed Vista as the third OS on my business laptop. Of course, when I did, it blew away my MBR and my computer forgot all about grub. Then I couldn’t boot into Ubuntu and I was not a happy camper. Although truthfully, I expected this to happen, it being a Windows install. Fixing this is surprisingly easy. Follow this tutorial to see how to do it:

Boot into the LiveCD.

Open a Terminal Session and become root by typing this:
sudo su -
Now let’s launch grub.
We’re going to find out where Grub should boot from.
find /boot/grub/stage1
Your output should look something like this (it’s what I get) (hd0,2). That’s your hard drive/partition. Once we’ve got that info, we can tell grub where your root directory is, and where the MBR should be.
root (hd0,2)
setup (hd0)

Now all that’s left is to quit grub and restart
shutdown -h now
Once your machine restarts, you should see the grub menu again and you can go on your merry way.