Without a little more detail I can only give a couple of guesses.
Which particular wifi shield as there are LOTS out there so a direct LINK would be helpful.
Which sketch exactly as again there are quite a few variations and does that sketch match the shield ? (LINK please)
Are you trying via a web address or an IP address ? Connections to IP's are usually easier.
Do you have any security software or policy's in place that may prevent a connection ?