Most networks don't allow open proxies. If you aren't using a proxy, then it's possible that someone that was using the IP address that you're currently using (which may have changed when you rebooted), was.
You should've received a more detailed messaage including a website to contact to have your IP address removed.
Please try reconnecting, and when you get the disconnect message, copy & paste it into notepad, or here directly, so that we can see the exact message that you're getting.
