If you do not have a link light on your network interfaces, they are not up and will not be able to communicate with the network. Your LAN and WAN interfaces both must have link lights.
If you do not have a link light on one of your network interfaces, there are a few potential causes and things to check.
Ensure the network cable is snugly plugged in on both ends. Unplug and replug the cable to ensure it is properly seated.
Try a different cable.
Make sure you are using the appropriate type of cable.
There are two types of standard Ethernet patch cables, straight and crossover.
Used to attach devices like computers, routers (ones like Cisco, not counting most DSL and cable routers/modems), servers, printers, firewalls, and other devices with Ethernet cards into a hub or switch.
Used to connect one hub or switch to another hub or switch, or connect a PC directly to another PC, or a firewall directly to a PC, etc. (No longer needed with gigabit ports, as they are all autosensing MDI/MDIX)
Make sure you are using the appropriate cable type for your situation. If you are unsure of which cable is required and do not get a link light with a straight cable, try a crossover cable.
If none of the above apply and you still are not getting a link light, verify functionality of both pieces of equipment by trying other devices. If you cannot get a link light on a network device no matter what you plug it into with any kind of cable, the device has a bad Ethernet port.
Note that some gigabit network cards do not properly support "Green" Ethernet standards and will never link on a "Green" switch. Changing to a non-green switch may fix this.