I would suggest you to use disk utility to verify if you still have the partition where you installed windows 8 or if you have formatted windows 8 partition. If disk utility is not available among you apps, you can download it from software centre.
If windows 8 partition does not exist, then you will have to install windows 8 or if your system have build in recovery or restore tool, you can restore back windows 8. Now if you are restoring back your system, you will loose the Ubuntu installation.
Or if you are installing a fresh copy of windows 8, you can partition your hard drive and set the desired amount of space for windows 8, install windows and and restore grub.
Hope this might help you.