this sounds like a hardware configuration error/resource conflict. we need to know your exact nic model. if you are uncertain, post back the output of:
dmesg | grep eth
once you know this information, go to
http://www.scyld.com/community.html (link edited) and find out what module is supposed to load with your card.
once you know the name of the card and its driver, find out if the driver had loaded:
lsmod | grep your_module_name_here
if your module is loaded, let us know if your card shows up in:
cat /proc/lspci
ifconfig -a
if your module is not loaded, try
insmod your_module_name_here
or
modprobe your_module_name_here