Fixing a broken grub2

For some reason every upgrade of Ubuntu destroys the working grub installation. This is how to fix it again.

First boot through rescue CD. Then to find out which partition your linux root system lies on do the following

fdisk -l

Mount it assuming /dev/sda3 is your root system

mount /dev/sda3 /mnt

Then set up a chroot in that root system and reinstall grub2

for fs in dev sys; do mount --bind /$fs /mnt/$fs; done
chroot /mnt
mv /boot/grub/grub.cfg /boot/grub/grub.cfg.old
grub-mkconfig > /boot/grub/grub.cfg
grub-install /dev/sda
exit
for fs in /mnt/dev /mnt/sys /mnt; do umount $fs; done

Reboot