I'm not sure how it happened, but I lost the "windows 7" entry from my Grub2 screen after updating ubuntu.
How can I add it ? I knew how to do it in grub legacy, not in grub 2
You could try installing grub-pc tools to automatically detect all available operating systems
and then update grub2 for your next restart
if Windows is correctly installed it should now be added to grub.cfg
This article seems to answer the question :