You can boot with the CD 1 of Slackware, mount your harddisk, chroot to it, run liloconfig and install it to the MBR, that will hopefully reinstall a working copy of LILO.
To do that issue the following commands:
mount /dev/hdXX /mnt
Replace hdXX with the device where Slackware is.
Also, probably your filesystem may got corrupted so before this commands, do: fsck /dev/hdXX