On my last Slack 9.1 installation I had the same problem.
I tried to install Lilo and it failed.
You can boot from Slack Boot CD - CD-1, and to pass to CD kernel to mount root as /dev/hda2
Than you'll have working Slackware and you can edit lilo.conf to fix the problem.
Check if the name of the boot image contains spaces. If it contains you have to remove them, because lilo install fails.
After fixing lilo.conf, just run lilo, and reboot
Everything has to be alright.