When GRUB is corrupted, your PC can't boot into Linux. Here's what I did:
First, grab a copy of the latest Ubuntu LiveCD and boot it. In my case, I made a startup USB thumb drive and booted with it. It boots a bit faster.
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 to be mounted as well. 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 and some other things needed in the chroot
$ sudo mount --bind /dev /mnt/dev
$ sudo mount --bind /proc /mnt/proc
$ sudo mount --bind /sys /mnt/sys
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. If you have a separate /boot partition you need to mount it first!
$ update-grub
To install GRUB 2 to the MBR, next you need to run grub-install /dev/sda
$ grub-install /dev/sda
If you encounter any errors, try grub-install --recheck /dev/sda
$ grub-install --recheck /dev/sda
Press Ctrl+D to exit out of the chroot.
Once you exit back to your regular console, undo all the mounting, first the /dev and others
$ sudo umount /mnt/dev
$ sudo umount /mnt/sys
$ sudo umount /mnt/proc
Now you can unmount the root system. (But if you have a separate boot partition which you mounted earlier, you have to unmount this first, or you will get a "device busy" error message.)
$ sudo umount /mnt
And you should be free to restart your system right into GRUB 2 and then into your system installation.
If you had alternate OS entries, update-grub might say "Cannot find list of partitions!". Ignore it and continue - once you can boot into your linux installation, do so and then rerun update-grub and grub-install /dev/sda as root.
Reference:
https://wiki.ubuntu.com/Grub2