I have come across this case a couple of times where for some reason after operating successfully for some days or weeks all port 80 traffic starts getting blocked by a 2wire router.
This occurs when the router is connected to the computers through a switch, as opposed to all computers connecting directly to the router.
In order to resolve it open the router management diagnostic console (MDC) by going to the router address /mdc
Under Local Network go to Configure
Check OFF Enable Router behind Router alert