Can you check your firewall configuration. In production environment most firewalls are configured to repond and let through only specific traffic (specific users or MAC adress). The firewall will simply reject (and not respond) to any other traffic.
It is the case in my network, unauthorised user simply can't get out to the internet and won't get any kind of response when pinging the firewall. When you tried with other working computer did you use your own username or used the account of someone already logged in? Would also help to know what kind of firewall you have...ie Watchguard, Cisco or other kind.