Step-by-step – Reinstall GRUB on Ubuntu 18.04 Guide

Fixing Grub

2. 2. Booting Live OS

   First, Boot from a Live Ubuntu OS
   On a PC or Virtual Machine you may need to Enter BIOS and Set the ROM/Drive as the First, loading Device.

3. 3. Chrooting Ubuntu

   Now to Chroot Ubuntu OS
   First, switch to the Admin Prompt:

   sudo su

   Now make the Target Directory with:

   mkdir /mnt/chrootdir

   Then Mount the main Partition:

   mount /dev/sda1 /mnt/chrootdir

   And the required Directories:

   for dir in proc dev sys etc bin sbin var usr lib lib64 tmp; do
       mkdir /mnt/chrootdir/$dir && mount --bind /$dir /mnt/chrootdir/$dir
   done
   

   Last, to Chroot simply play:

   chroot /mnt/chrootdir

4. 4. Fixing GRUB

   Now to Reinstall GRUB on Ubuntu
   Run:

   grub-install /dev/sda

   And again:

   update-grub2

   Next to Exit Chroot Env:

   exit

   Finally, Reboot into your Restored System

   reboot now

   Thanks to askubuntu.com #254491.