Hrm, not exactly that error. Did you install lilo to the MBR or just to a bootable partition? I don't think it can be overwritten in the MBR but I'm not really sure.

Solutions that I would try are:
boot from the slack disk and at the boot: prompt type in:

bare.i root=/dev/hda1 noinitrd ro
*change hda1 with your / partition.

see if that works and if it does you can reinstall lilo from the setup menu.

Use a gentoo boot cd or a live! distro to go in and mount the partitions and repair whatever files need to be replaced manually.