are your DNS settings entered properly in your router's setup??
go to your network connection's propertites, TCP/IP > properties>make sure you have some DNS IPs on there. if you dont please enter them in. if they are setup for auto please go into your router's setup and make sure that it has gotten the DNS info. If not, please enter that manually into your network connection.
you may have to go into the setup and do a DHCP realease/renew in case it "lost" the DNS info.
If you do not connect with a router please disconnect the cable modem and try to see if it got the right IPS/ DNS info.
You may have to wait an hour or so for it to give you a new IP/DNS info.
I hope this has been helpful, got any other questons, Please feel free to ask!