Hot days lately? I just had an ethernet card blow up with a similar problem, but its rare at best.
Try running "ping
www.google.com -t" from the Start > Run menu on your computer. Get ping times back? That means something like Windows Firewall or NetNanny, etc.. is blocking websites.
Try pinging your ISP and your router. If you can't hit your router, then you know the issue is your computer or the cable.
If your ethernet port on your motherboard/card has a light, make sure if its on. If its not, you could try swapping out the cable.
Try connecting the laptop via a wire, if you can. Then you'll know its your computer-- hardware or software.