You probably installed lilo to / or /boot rather than the
MBR ... so the box thinks there's no OS on the machine.
If this is the case ...
To fix it, boot with the install-cd, select the appropriate
kernel, and tell it to use the partition you installed
Slack to ... e.g.
bare.i noinitrd boot=/dev/hdaX ro
where X is the root partition you installed to.
That should boot your installed Slack and let you
modify /etc/lilo.conf to have boot=/dev/hda
rather than e.g. boot=/dev/hda1 ... save that, then
Reboot, and voila!