There are times when you inadvertently overwrite your Master Boot Record. The end result being that you are unable to boot into Linux. This is especially true when you are dual booting between windows and Linux OSes. Once when I was working in Windows XP, I accidentally clicked the hibernate button instead of shutdown. And windows somehow overwrote my MBR which housed the GRUB boot loader. At such times, it pays to have this cool tip at hand.
This is what you do to restore the GRUB boot loader when faced with the above problem. First you need a Linux distribution CD. If you are using Fedora (RedHat) then the first CD is sufficient. But you may also use any of the live CDs like Knoppix, Ubuntu Live CD and so on.
With Fedora CD
Boot your computer with the first CD of Fedora in your CD drive (You have to enable your PC to boot from the cdrom, which you can set in the BIOS settings). At the installation boot prompt that you get, enter the following command:
boot: linux rescue
… and press Enter. The installer will ask you a few questions like the language you would like to use, the type of keyboard etc. Then, if you have linux previously installed on your machine, the Fedora installer will automatically detect it and mount it in the /mnt/sysimage directory. Once the linux partition is mounted, you are dropped into the command shell prompt. The next step is to make your newly mounted directory the root (or parent) directory. This you do by running the chroot command as follows:
# chroot /mnt/sysimage
Now you are in the shell with respect to the parent directory which is the linux partition on your harddisk.
From here, the steps needed depends on which bootloader you are using. You have to have a fair idea what is the device node of your harddisk partition housing your MBR. In most cases, it is /dev/hda if you have an IDE harddisk. But if you have a SCSI harddisk, it will be /dev/sda.
Execute the following command :
# grub-install /dev/hda
… to install GRUB boot loader on to your MBR. And then type exit to reboot the machine. Now your GRUB boot loader is fixed.