You might start by giving us the layout of your configuration (wireless, what connects to your computer, etc.). The problem you are describing, usually occurs if the network adapter cannot make a connection to the DHCP server to get a valid IP address.
Until you post back, perhaps unpower your network devices, then repower starting from the broadband connection then back from that.
For Instance, unpower everything, then repower the Cable modem, Router, then your computer.