Could be anything. You've been blocked by the company's firewall, you aren't going via the proxy, the proxy is blocking you, DNS issues...could be a number of things.
Can you ping the Internet? Try from command prompt:
ping 8.8.8.8 (if this works you have Internet connectivity)
ping www.google.com (if this works then you may have browser issues - try clearing the browser cache)
If both work, and you're sure your PC isn't at fault, then it's probably proxy or firewall related.
Have you reported to IT Support?