ping localhost or ping 127.0.0.1
If it doesn't reply, your NIC is probably not installed or dead.
Make sure you have your drivers installed.
If you plug it into a network and you don't get a light on the NIC (and you have a hub/switch/router on the other end of that ethernet cable) it'll probably be dead.
Try another NIC if you can. If that works, your onboard one is dead.
Check to see if your Onboard NIC is disabled in BIOS.