So I'm guessing you didn't create a boot floppy?
If not, boot with the CD, and hit ALT+F2. That should get you to a terminal. I'm not sure, because I've never used Slack. Just try all of the function keys.
Mount the partition that you created as root to a directory. It might look like this:
mount /dev/hda1 /mnt
If all goes well, look in /mnt/etc/lilo.conf (replace /mnt with whatever you mounted the drive to). Depending on what tools that distrobution loads with its CD, the editior could be vi, pico, or just edit.
Post the contents here and we can get working.