Windows has its own boot loader and it will overwrite the Linux one. Unlike
Linux, Windows does not like to share and will not detect Linux as an OS and
give it a boot option in its boot loader.

To fix your problem you need to use the Live Ubuntu CD to boot from and then
rewrite grub following these instructions:
www.howtogeek.com/.../reinstall-ubuntu-grub-bootloader-after-win\
dows-wipes-it-out/