Well, it is most likely Ubuntu failed to install GRUB into the MBR since you can't boot Windows

Well, it is most likely Ubuntu failed to install GRUB into the MBR since you can't boot Windows anymore.

The solution would be to manually install GRUB into the MBR. My suggestion would be to use this tool called Boot-Repair. you can find a tutorial in this page. After you reinstall GRUB, you'll most likely be able to boot both on Windows and Ubuntu.