Here are a couple ways of doing it and hopefully one of them will work for you:
Boot with a Knoppix CD (or some other live-cd), or use a rescue/boot floppy like Tomsrtbt (
http://www.toms.net/rb/ ); determine your root partition and mount it to a temporary mount point; then chroot into it; then reinstall GRUB:
sudo mkdir /mnt/temp
sudo mount /dev/hda1 /mnt/temp
(change /dev/hda1 to match your own root partition)
sudo chroot /mnt/temp
sudo grub-install /dev/hda
or...
determine your root partition, then mount it "dev" option enabled with write permissions. If the filesystem isn't mounted you'll need to mount it like this (be sure to change "hda1" and "hda" to match the location/device in your own system):
sudo mount -o dev,rw /mnt/hda1
if it's already mounted, remount it like this:
sudo mount -o remount,dev,rw /mnt/hda1
now restore grub like this:
sudo chroot /mnt/hda1 grub-install /dev/hda
if it doesn't work using chroot, try remounting as outlined above and do:
sudo grub-install -root-directory=/mnt/hda1 /dev/hda
Kanotix has an option to "Restore Grub", simply boot with the livecd and choose restore grub.
