You can also use Boot-Repair, it will auto repair and fix the existing OS boot.
Type this in terminal:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update && sudo apt-get install -y boot-repair && boot-repair
Boot-Repair can be installed & used from any Ubuntu session (normal session, or live-CD, or live-USB). PPA packages are available for Ubuntu 10.04, 10.10, 11.04 and 11.10.
Using Boot-Repair
gksu boot-repair
Then try "Recommended repair" button. When repair is finished, reboot and check if you recovered access to your OSs.
The option "Recommended repair" will auto search existing boot OS and will auto fix the boot records.
For more option, you can set, look at the GUI interface:
https://help.ubuntu.com/community/Boot-Repair