Might be good to follow basic network tools like Ping
go to start, run, type 'cmd", at dos prompt, type "ipconfig
/all"
take note of the IP address, subnet mask, gateway address, are they
assigned correctly from the router itself else, try "ipconfig
/release" and then "ipconfig /renew"
try ping the gateway address as well (gateway = router in this
case)
if there's reply from gateway, probably is the router to modem
connection.
If that's the case, check the WAN IP of the router, might not be
getting proper address from the modem.
My 10cent worth of information.