Maybe your network adapter is disabled?
(I am assuming you are using Windows 7)
Open Network and Sharing Center
In the left pane, select "Change adapter settings".
Right click on the connection and enable it.
If that doesn't work, unplug your network cable and check for:
-holes in the cable
-corrosion on the pins for the plug head (looks like greenish / whiteish dried battery acid) (this is caused by moisture)
-loose connection between the cable and the plug head (maybe if you accidentally pulled on the cable instead of the plug head)
If there are holes or the plug head is loose, you're better off buying a new cable.
If there is corrosion on the plug head (and you do not want to spend money on a new one), dab the corrosion with medical alcohol and dry it off. (it should remove the corrosion)
If it is not related to the physical connection of the cable or the software/firmware side of the network adapter but rather the physical state of the network adapter, I'd say your choice is good money-wise/performance.