I would check in device manager (control panel, system, hardware tab, device manager, network adapters) to see if your network card isn't installed properly (indicated by an exclamation point or red circle/white X). If your network adapter is installed properly, right click on the driver and make sure it's enabled. If it is enabled and installed properly, then I would download winsockxpfix (free download, use only if you have XP) and run that.
If your driver isn't installed properly and you haven't made any changes to your computer recently that could have affected that, try to reinstall the driver. If you are unable to install the network driver, then I would suspect a defective network card.
Hope that helps
