[You need to use "root" account to do following]
# dd if=/dev/hda of=/dev/fd0 bs=512 count=1
This makes an exact copy of the MBR of the first hard drive (hda - you need to replace this), copying it to a floppy disk. You can boot directly from this floppy, and see your old boot menu. You can restore it by switching the “if=” and “of=” (input file, output file) parameters.
If you don’t have a floppy drive, you can back it up to a file with this:
# dd if=/dev/hda of=/home/nik/boot.mbr bs=512 count=1
Then you can boot into a CD-ROM distribution such as Knoppix, or often use your Linux distribution’s installation CD to boot into rescue mode and restore it with:
# dd if=/mnt/hda5/nik/boot.mbr of=/dev/hda bs=512 count=1
(you’ll need to find and mount the partition containing the directory where you backed up the MBR).