# Boot on a live CD and open a console. sda1 is supposed to be the partition containing the OS
# 1/ Restore GRUB
mount -t proc none /dev/sda1/proc
mount -o bind /dev /dev/sda1/dev
chroot /dev/sda1/ /bin/bash

/usr/sbin/grub
grub > root(hd0,0)    # 0->a, 1->b ...      #1->0, 2->1 ...
grub > setup(hd0)
grub > quit

# 2/ Check if partition is bootable
fdsik /dev/sda
fdisk > p      # print the partition table
fdisk > a      # toggle a bootable flag
fdsik > 1
fidsk > w

# 3/ Reboot !! That's done