source : https://wiki.ubuntu.com/Grub2

Recover Grub 2 via LiveCD

• First, grab a copy of the latest Ubuntu LiveCD and boot it.

• Open a terminal and type

$ sudo fdisk -l

• Now, you need to remember which device listed is your linux distribution, for reference, /dev/sda1 will be used. Now we need to mount the filesystem to /mnt

$ sudo mount /dev/sda1 /mnt

• If you have /boot on a separate partition, that need's to be mounted aswell. For reference, /dev/sda2 will be used.

$ sudo mount /dev/sda2 /mnt/boot Make sure you don't mix these up, pay attention to the output of FDISK

• Now mount the rest of your devices

$ sudo mount --bind /dev /mnt/dev

• Now chroot into your system

$ sudo chroot /mnt

You should be chroot'd into your system as root, you can now run commands as root, without the need for sudo.

• Now you need to edit the /etc/default/grub file to fit your system

$ nano /etc/default/grub

• When that is done you need to run update-grub to create the configuration file.

$ update-grub

• To install GRUB 2 to the MBR, next you need to run grub-install /dev/sda