Are you connecting via wireless? It sounds like your router and computer won't talk to each other properly. The first thing I'd check is that you haven't changed your firewall settings inadvertently. Try disabling whatever firewalls you have running and see if that works; if it does, try reconfiguring the firewall. If not, there could be some conflict with your wireless adaptor, about which I don't know enough. If you could give a bit more information about your system, your equipment and operating system, some of the more informed guys on here will most probably be able to help you.