Lots of possibilities.
Try switching the whole lot of at the mains - wait a minute or two before switching back on in this EXACT order:
Modem - wait a minute...
Router - wait...
Pcs and laptop.
Any good now?
If not - what router security are you using? WEP, WPA? If not WEP, try chaning to WEP.
How have you got the router configured? Mine has MAC filtering so unless I tell it to, it won't let any strange pcs or laptops connect.
I have mine configured to only allow 2 pcs to connect at any one time - could that be the problem?
Could it be the firewall on your laptop? Try temporarily disabling it.
If all else fails, reset the router to default (hold in the little timy button for 10 seconds). This means there will be no security on it whatsover, so be careful until you have configured it.