Listen to Reboot, do a full hard reset. Hold the reset button in for a full ten seconds, not just one. (One second will do just a soft reset.)

A much more rare problem but it has happened to me. That RJ45 cable from your modem to the airport extreme can flake out. Since the airport signal is good swap the cable out with a new one. I've had one go bad just looking at it.

And last but not least, did you pay your bill? Comcast cuts internet and not cable when the bill is overdue. You get them on the phone and pay by credit card it is back up in minutes. I have done this a couple times since I have two comcast bills and when I moved them to autopay they keep screwing it up. (I still don't have them on autopay.) You tell them something like that and they never tag you for a "hookup service fee".

Come to think of it, that's a third humongous company that I have always had the best service with. Comcast phone service people are the best. Right up there with BOA and Chase. cool