OK - I take it you created a bootable setup disk to run the install from. You can boot up using this disk and follow the instructions to boot to an installed Slackware system. At the lilo prompt of the boot disk hit tab to enter lilo command prompt and enter :
huge.s root=/dev/sda2 rdinit= ro
where /dev/sda2 is the partition you installed Slackware to. Then check the file /etc/lilo.conf to see it's all OK and as root run "lilo -v" to reinstall lilo. You should then be able to boot into the installed Slackware system directly.