It could be the network card, those PCI network cards break easily. I think I have replaced about 3 of them around the house.
But yah, before you go and buy a new one, try to install in another working network card, if that one works, you know it's the old network card problem. If not, check the CAT5 cord by either using a different cord, if that's not the case, try to restore the configurations for networking to default. Also, be sure to check your router that the IP of the current computer you were trying to get connected to is not blocked in some sort.
The other usual alternative is to restart the modem and the router.
Let me know if any of these help you.