Try booting from a bootable linux CD and doing the following:
# fdisk -l /dev/hda // locate the device containing your / partition (the example assumes it is located in the master position of the Primary IDE controller)
# mount /dev/hdaX /mnt // Once you find out which device contains your / partition, mount it somewhere
# mount -o bind /dev /mnt/dev
# mount -o bind /proc /mnt/proc // bind the /dev and /proc filesystems in the corresponding destinations inside the mountpoint of your / partition, just in case lilo needs them
# chroot /mnt // chroot inside your / partition
# lilo // rewrite lilo, according to the settings of it's configuration file, normally in /etc/lilo.conf
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