It sounds like you may have set your computer up with a static IP address or that your adapter has been disabled. Go into the Control Panel and then into Network Connections. Look for something with the name "Local Area Connection", if it is grey, right click on it and click "enable". If it is not grey, right click on it and go to "Properties". In list there named "This connections uses the following items" scroll down to the bottom where "Internet Protocol (TCP/IP)" is and click on it. Then click "Properties", make sure that "Obtain an IP address automatically" is checked for both IP and DNS. Then click OK and then OK again.
Let me know if this fixes your problems.