I've done it once. I can't remember the source of the info, but here's my notes:
Graphical Method – Boot Repair
Boot Repair is a graphical tool that can repair GRUB2 with a single click. This is the ideal solution to boot problems for most users. If you have the media you installed Ubuntu from, insert it into your computer and restart.
If you don’t, download a Ubuntu live CD and burn it to a disc or install it on a USB flash drive. You can also download a dedicated Boot Repair live CD. After booting into the live Ubuntu environment, open a terminal from the Dash and run the following commands to install Boot Repair:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair