How to boot to a Ubuntu 12.04 installation which is not listed in grub2 anymore?

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http://askubuntu.com – I have a computer with 3 hard drives on it. I have Ubuntu 12.04 on my newest and biggest hard drive and Windows XP on my 2nd hard drive and a old version of Ubuntu 7.04 on the third hard drive. Everything was working perfectly and I was using Grub to choose between the different systems. I was using Windows XP and it automatically did some upgrades and the next time I tried to use Windows XP, it would not not boot up. So I tried using the Windows XP CD to repair it and I used a dos command like a dltmbr or something similar to it and when I tried to boot into Ubuntu 12.04 it would not boot! (HowTos)