Have you ever installed Grub-Customizer? Yes, it's a nice tool for rearrange our boot system and gain some extra space in the discs. Very usefull though!
But, as i recomende that you install this usefull app, at the same time i advise you to be carefull, because any distraction and your dual boot simply could go away!
And how can we fix this as an easy way?
Boot your cd (as you did before) with your Ubuntu (if you have latest version installed in your pc it doesn't matter if you boot another version of ubuntu. If you have Natty 11.04 you can simply use your 10.04 cd...)
try the experiment mode (you don't need to install ubuntu again!)
Then install (as you have made in your system) Grub-Customizer packages (sudo add-apt-repository ppa:danielrichter2007/grub-customizer && sudo apt-get update)
in update manager select the option "open source code software (Universal)"
In the terminal again, install the repository packages (sudo apt-get install grub-customizer)
Acess to Grub-Customizer and select your root. Then your boot option (in case you have dual or triple boot) will appear and you can select again and treat a big headache...
Hope you liked it!