Some solutions :
Mount your first slack CD -e.g under /mnt/cdrom/
- type "makebootdisk " under root account and choose "lilo" choice and take one kernel of the slack CD for making a boot disk on floppy with the kernel choosen.
- copy from the slack CD, bzImage, config et System.map.gz from a good kernel for you - e.g from /mnt/cdrom/bare.i - to /boot. and check /etc/lilo.conf to set the good values and run lilo to update. Do not forget to gunzip System.map.gz.
- If you have compile a kernel by yourself, compile it again et reinstall the result under /boot make changes to /etc/lilo.conf et run lilo.