Network interface card (NIC) is not seated properly, or if 4-port Ethernet card, wrong interface connected.
Check to see if the NIC has pulled away from the motherboard. If so, reseat the NIC firmly in its slot. For instructions, refer to the documentation for your card.
After the system reinitializes the interfaces, the "enabled" LED on the NIC should go on. If this LED remains off, the system detected a processor hardware failure. Replace the failed NIC.
If you are using a 4-port Ethernet card, verify that the port you plugged the cable into is the correct one. If the cable is plugged into the first port on the card, try plugging it into the last port. Also, use the show interface command to verify that the interface is not shutdown.