I recently installed Fedora 12 on my desktop alongside Ubuntu Karmic and Windows 7, however doing so I overwrite my grub which meant my Ubuntu system was hidden. To fix this is pretty simple..
Insert your Ubuntu Live CD
Click “Try Ubuntu without any changes to your computer”
Once loaded click Applications > Accessories > Terminal
sudo fdisk -l
#That’s a lower case L
This displays all partitions on your system, in my case the installation of my Ubuntu system was /dev/sda5 so replace this for yours in the rest of the code.
sudo mount /dev/sda5 /mnt sudo grub-install --root-directory=/mnt/ /dev/sda
#thats a double dash before root-directory, wordpress keeps changing it to a long dash, Ill fix it sometime..
If all goes successfully you should be done, try rebooting and see if your Ubuntu shows up.
Thanks to Whiblog in the comments this also works in Ubuntu 11.04:
“same thing except the last line : sudo grub-install –boot-directory=/mnt/ /dev/sda”