You are probably missing at least one key file.
If you can't get to the OS sellection screen, in your case, the Grub screen, it means that those files are missing or corrupted.
From your Linux Ubuntu installation medium, reinstall the Grub interface. If those files are accessible from Windows: Open a console. Set ALL files in the root folder as Read only, System AND Hiden.
The command needed is: attrib *.* +s +r +h
As some files don't have any extention, also do this command: attrib * +s +r +h
This will reduce the risk of having those files accidently deleted or altered.
You can probably set restricted access for the key files from the Linux environment. My lack of Linux knowlege prevent me from telling you exactly how.
Both OSs are probably intact as well as all your user files and installed applications.