Repair the GRUB from live CD or USB stick

  1. Insert the bootable device and boot into Try Ubuntu mode

  2. Find out you root partition by typing: sudo fdisk -l or sudo blkid

  3. Mount you main partition: sudo mount [/dev/sda2] /mnt

Note: If you have separate partitions for /boo, /var and /usr, repeat steps 2 and 3 to mount them to /mnt/boot, /mnt/var and /mnt/usr respectively

  1. Bind-mount pseudo-filesystems: for i in /sys /proc /run /dev; do sudo mount --bind "$i" "/mnt/"$i"; done

Note: A bind mount is an alternate view of a directory tree. Classically, mounting creates a view of a storage device as a directory tree. A bind mount instead takes an existing directory tree and replicates it under a different point. The directories and files in the bind mount are the same as the original.

  1. If Ubuntu was installed in EFI mode, use sudo fdisk -l | grep -i efi to find your EFI partition and then mount it: sudo mount [/dev/sda1] /mnt/boot/efi

  2. sudo chroot /mnt

  3. update-grub

If this doesnt't fix the GRUB, go on:

  1. grub-install /dev/sda; update-grub

  2. I fthe EFI partition ha changed, you may need to update it in /etc/fstab accordingly

  3. sudo reboot

  4. At this point, you should be able to boot normally (remember to unplug the bootable media).