im willing to bet you either installed the wrong kernel module or you didnt install it at all.
a simple /sbin/ifconfig will tell you if ethX is functioning. if it is not, try modprobing the driver for it. if you do that and it doesnt work, just make doubly sure you compiled in the right driver when you recompiled the kernel. possibly boot up your old kernel, and then do a cat /proc/modules to see exactly which modules are being used. then remember these when you recompile your kernel next time.