First, ping your IP adress. If it is not responding, reinstall TCP/IP.
Second, ping your gateway. If it's not responding, the gateway is out of service or the IP adress and specially the net mask of your workstation and the gateway don't match.
Third, ping the IP of the gateway's network adapter which is conected to the Internet. If it is not responding, the gateway is out of service, forwarding disabled or a kind of firewall is blocking you on the gateway (bad configuration).
Fourth, ping the gateway's gateway IP (the end of your conection to your ISP). If it is not responding, contact your ISP.
Last, ping different URLs in the Internet and good luck!