Try a couple of things:
1) This could be a network connectivity issue. So unplug your network cable and reboot. If you are able to, you'll need to tinker with your network settings.
2) If 1) does not work, try booting into safe mode both with networking and without networking (am not talking about the cable but about the safe mode options presented when you press F8 or Del), see which one works. You might then need to do a Clean boot procedure to isolate the culprit driver