Power off your cable modem, and router (unplug them both).
Wait 30 seconds. Plug the cable modem back in. Let it go through the whole process. Make sure you have the Cat5 cable going from the modem to the LAN port on the router- usually sets off to the side by itself. No plug the router in. Make sure your Cat5 connections to your router are secure to the other ports, running directly to the computers. If you have XP, it's not even necessary to restart the computer. It should get an IP automatically. If it's any OS besides XP, you will have to release and renew.
Windows 2000- Start>run>cmd>OK>ipconfig /release all
Then type ipconfig /renew all.
Windows 98/ME- Start>run>winipconfig
Hit the release button. Then the renew button. Also make sure your network card is selected.
If you still get limited or no connectivity, you should check your router settings, and make sure it is assigning more than one IP. Why it wouldn't is beyond me, but you may want to check.