Tente isto:
Inicialize o Ubuntu Live Usb-Dvd.
Abra um terminal
Pressione Ctrl + Alt + T
Execute:
sudo -i
mount /dev/sda2 /mnt
mount /dev/sda1 /mnt/boot/efi
grub-install --boot-directory=/mnt/boot --bootloader-id=ubuntu --target=x86_64-efi --efi-directory=/mnt/boot/efi
reboot