You did not upgrade your kernel to 2.6 first.
You should read the instructions in the Slackware 12.0 CHANGES_AND_HINTS.TXT
Especially the first step in the upgrade procedure which is described in detail there:
Install a 2.6.18 or later kernel (if you are using the default 2.4 kernel
from Slackware 11.0) - there are kernel packages available in the A
series that should work fine. Note that the generic* kernels will
require an initrd, and 11.0's mkinitrd is unable to build a suitable one,
so you need to choose one of the huge* kernels for now. There is a
discussion later in this document about whether to use the SMP-enabled
kernel or not.
At this point, use "installpkg" to install the kernel rather than
"upgradepkg" so that you will still have a working kernel if the new one
doesn't boot. If you were not already running a 2.6 kernel, you will
need to reboot before doing anything else. Before you reboot, you will
need to make sure the old and new kernels have an appropriate section in
/etc/lilo.conf, and then run /sbin/lilo.