http://www.howtogeek.com/howto/ubuntu/reinstall-ubuntu-grub-bootloader-after-windows-wipes-it-out/
1) Boot off the LiveCD
2) Open a Terminal and type in the following commands
sudo grub
root (hd0,0)
setup (hd0)
exit
Reboot (removing the livecd), and your boot menu should be back.
