Yeah, there are several places where you could be having problems, and you just sorta need to look around a bit to see what did and didn't happen:
- Did the kernel get built (or installed from a package)?
- Is there a file on /boot?
- Is there a new entry in the boot-loader menu (e.g. /boot/grub/grub.conf)? Or in the case of LILO, has it been re-run?
Obviously, I'm not a gentoo'er myself, but this is the general procedure to follow...