you can mount the old linux, then chroot to it's root directory, and run lilo again.
any old linux boot disk will do - I recommend tomsrtbt:
once you've booted to a boot disk, or some form of linux, create a directory to mount your existing install, like so:
mkdir -p /mnt/old_linux
then mount the partition you want to make bootable again (you'll need to know which partition it was on (for example, /dev/hdb1):
mount -t auto /dev/hdb1 /mnt/old_linux
(you can change 'auto' to whatevet actual fs type it is, usually ext2 or ext3)
then chroot to that (this will change the root directory from the boot disk's / to /mnt/old_linux) and rerun lilo:
chroot /mnt/old_linux /sbin/lilo
that should do it!
(btw, I was going from a fuzzy memory, so I hope someone will correct me if I'm off! TIA!)