Provided that you have installed the correct drivers and that you don't have any software/malware blocking the connection and another computer works connected to the same ethernet line than something probably fried your nic and you'll need to buy a new one.
Vem's suggestion is good as well, however it seems like you have something more going on. Regardless try that and then continue to troubleshoot until you feel comfortably sure that it's the nic not working and then order a replacement.