Repairing/Restoring a missing GRUB2 bootloader
If you reinstall Windows or otherwise accidentally destroy your GRUB(2) bootloader, don’t worry. This is a fairly straight-forward problem to fix.
1) Boot from a live-cd or USB drive with the same or similar Linux distribution
2) Open a terminal and become a superuser. (“sudo su” for those that support it, “su” and the appropriate password for those that don’t)
3) Find the partition your operating system lives on, something like this:
fdisk -l /dev/sda
Disk /dev/sda: 32.2 GB, 32212254720 bytes
255 heads, 63 sectors/track, 3916 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 * 1 2581 20731851 83 Linux
/dev/sda2 2582 2647 530145 82 Linux swap / Solaris
4) Mount the partition in a temporary location, /mnt for example:
mount -t ext3 /dev/sda1 /mnt
5) execute the following commands and reboot:
grub-install --root-directory=/mnt /dev/sda
update-grub
6) Check to make sure your grub config is ok on reboot. If not, edit your grub config file and rerun the “update-grub” command.