This is a very common problem, with a couple of different causes. Do you have a boot disk? If you can boot off of that, check your /etc/lilo.conf and see where it is pointing to. As root type:
cat /etc/lilo.conf | less
and you should see a line that says:
image = /something/vmlinuz.2.4.blah
If this line isn't pointing to the right location than Lilo won't be able to fine the kernel to boot it.