how to repair grub and get ubuntu back after installing windows

First make sure that you have turned off fast boot from Windows power settings and disabled secure boot  from BIOS.

Boot from Ubuntu CD, open the terminal and run the following commands

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

After completing the installation you can launch it from System-> Administration-> Boot-Repair menu if you use Gnome, or search "boot-repair" in the dash if you use Unity.


Select the recommended Boot repair options as shown in the screenshot and follow the steps suggested by Boot Repair






Source: http://askubuntu.com/questions/88384/how-can-i-repair-grub-how-to-get-ubuntu-back-after-installing-windows


EDIT: Recently I upgraded from ubuntu 12.04 to  14.04. I faced similar problem and this solution worked like a charm !