If you uninstalled windows before install Ubuntu did you format the drive and wipe any recovery partitions? May be you can reset the PC to its original configuration and start again.
Does it have the windows serial number on the base?
You could download and make the windows boot media from MS click here click on the option to "install windows 10 on a different PC"
Format the drive to get rid of grub and put a clean version of windows on it (you may not even need the serial key as the machine should be registered on the MS serves for its original serial key.