If you killed your trusty grub (i.e. by new Install of Windows 8.. :/) you can repair it quite easily:

• Boot with an Ubuntu 12.04 Live CD
• CTRL + ALT + T

sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair

And click on the repair button! Done!

Source: http://askubuntu.com/questions/333483/how-to-restore-grub-after-deleting-it-from-windows-8-disk-manager