sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
isudo apt-get install -y boot-repair && boot-repairsudo add-apt-repository ppa:yannubuntu/boot-repair


I've had this issue since the beta version of ubuntu 16. I find that after the error shows up, if you reboot to the live cd and run boot-repair, the problem is fixed and ubuntu boots. To get windows booting again, all you need to do is run sudo update-grub