Try this to recover grub:
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1    Open the live version of Ubuntu (either brun the LiveCD or install the image to a bootable USB device using Startup Disk Creator)
2    Open terminal and run sudo fdisk -l to see where Linux is installed on
3    Run sudo mount /dev/sdxY /mnt where x is altter and Y a number you have found in the previous step
4    Run sudo grub-install --root-directory=/mnt /dev/sdx to install grub
5    Run sudo update-grub to update grub
6    Reboot