Without knowing how you installed Ubuntu it is hard to answer this.
The default choice in the Ubuntu installer is "Install alongside Windows", and it will shrink the size of the Windows partition (if there is room and it is not messed up) and then create a Linux partition and install there. If you did that, your windows is there, and you can boot into Ubuntu on your hard drive, and restore grub by clicking on the terminal then typing:
If you did not select the default "Install alongside Windows", you may have erased windows and replaced it with just Ubuntu. If you did, reinstall Windows, then install Ubuntu again in that order.
P.S. To be safe, you should always backup your drive before installing a new OS or new version of an OS.