Sometimes, a power failure or manual interruption during a Linux OS upgrade can render the machine as un-bootable. Here are some methods to recover from such events:

Method 1:

1. Ensure that the network cable is attached to the target machine and Internet access is available
2. Boot up the system using a live CD or a Ubuntu Installation USB Thumb drive
3. Initate the Terminal emulation program and run these lines:

rootPartition=$(df | grep /$ | awk '{print $1;}')
sudo mkdir -p /recovery/ubuntu
sudo mount $rootPartition /recovery/ubuntu
sudo mount --bind /dev /recovery/ubuntu/dev
sudo mount --bind /proc /recovery/ubuntu/proc
sudo mount --bind /sys /recovery/ubuntu/sys
sudo cp /etc/resolv.conf /recovery/ubuntu/etc/resolv.conf
sudo chroot /recovery/ubuntu
sudo apt-get update -y
sudo dpkg --configure -a
sudo apt-get upgrade -y
Method 2:

Although ‘Method 1’ is recommended, this alternative is about reinstalling grub2 in cases where dual boot corruption has been suspected. Here are the commands:

# Alternative A:

# Source: https://help.ubuntu.com/community/Grub2/Installing

# Discover root
ls
# Set environment prefixes
set prefix=(hd0,1)/grub # these values may vary on each system
set root=(hd0,1)
# Get into shell
insmod normal
normal
# eh...
linux /boot/vmlinuz-3.2.0-14-generic root=/dev/sda1 ro
initrd /initrd.img # select the correct image
boot
sudo update-grub
bootDisk=/dev/sdb # Define boot disk (not partition)
sudo grub-install $bootDisk
sudo update-grub

# Alternative B:

bootDisk=/dev/sda
bootPartition=/dev/sda1
sudo mount $bootPartition /mnt
sudo mount $bootPartition /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install $bootDisk
update-grub
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