I don't know what you have going there, but if you compiled the kernel when you are realively new to this stuff, my bet is that your boot loader is not setup correctly. You need to check to see if you are poiting to the correct kernel and see if the old one is removed or has changed names.
I don't know how gentoo does things, but if you can get to the cli, then look at your either lilo.conf or grub.conf and check to see if your information lines up with the correct vmlinuz for the kernel or maybe the init file, which can be called different things, eh.
So, log in, goto /boot and see what's there. Open another terminal and see what /etc/lilo.conf or etc/grub.conf look like. You can draw a lot from there without further explanation by me. See if you can figure out if the information in that file lines up with the new kernel that you build (/boot), I assume without problems. If not, change your settings to match up with the latest kernel in /boot and maybe init file.
If it's not that, then I wouldn't know unless I was there to go from each point to the next, I'm afraid. Sorry about that. But, that is probably the place where most folks will get lost, imo, on the first time of compiling their own kernel.
Hope this helps, and I hope you don't give up because of this gentoo thing. There are other distros, and in time, you won't blink twice at this type of situation, and you will know what's up or the direction to take next.