If 'ifconfig' only shows the loopback device (lo, 127.0.0.1) and nothing else, then it probably means the driver (module) for your nic isn't loaded. Try doing a search on www.google.com/linux
for the model of your nic card, one of the pages will probably mention the module name (like tulip or ne2000 or something). If you find the module name for your card, do 'modprobe module_name'. with, you know, module_name being tulip or something. If you don't get any errors, try ifconfig again, hopefully eth0 will have shown up.