Experimente a sessão ao vivo, no USB ou cd, para reinstalar o grub-efi. Abra um terminal e insira os seguintes comandos:
sudo mount /dev/sda# /mnt
Replace the hashtag in /dev/sda# with the number representing the partition where you had Ubuntu 16.04 installed. You can use gparted to do it.
sudo mount /dev/sda# /mnt/boot/efi
Replace the hashtag in /dev/sda# with the number representing the efi partition. It's usually /dev/sda2.
sudo mount -o bind /dev /mnt/dev
sudo mount -o bind /proc /mnt/proc
sudo mount -o bind /sys /mnt/sys
sudo mount -o bind /run /mnt/run
sudo chroot /mnt/
sudo apt-get install --reinstall grub-efi
sudo update-grub
reboot , para perceber se funciona.