Make sure you have all of your drivers installed for your NIC or onboard NIC. How many Ethernet ports do you have? Make sure you have it in the right one. After that, check the other end of the cable. Is it pluged into a switch, router, or modem? Make sure that the switch/router/modem has power. Are there any blinking lights? If it's a modem, make sure the coax cable is plugged in, if it's a router or whatever, make sure that internet is coming from somewhere (trace it back until you hit the modem). If there are no lights flashing on your modem (I think two should be flashing, and two steady on most), then unplug it, wait a few seconds, and plug it back in. If the lights don't start to flash after about a minute, then it seems to be a problem on their side. If they are flashing, then a cable is loose somewhere, or this is something more that needs to be done to your computer.