The first step is make sure this is not a hardware problem; if it is a hardware problem, all the software troubleshooting in the world won't fix it.
First find a cable that you know works, either because it's in use or because you have tested it with a cable tester.
Make sure the cable is properly seated in the ports at both ends. If the far end is plugged into a router or hub, plug it into a port which you know works (perhaps one currently serving another device).
If it still doesn't work, connect the cable to another computer where you know the ethernet port works and test it. If the cable works in those circumstances, it may well be the port on the eithernet card (or the ethernet card itself).