It's a problem with your machine or your router, because the Airport showed a explanation point on the Mac.
If it's a problem with Comcast, then it's still going to show a good connection to the router, but your web pages won't load.
To test this, get the Ethernet cable by disconnecting your router from the modem.
Connect the Mac to the modem directly, then pull the modem power and wait 30 seconds, plug it back in.
You'll connect to the ISP directly and your Mac should work fine, if so then it's a router problem and not your Mac.
Reset your router in it's entirely, read it's manual and set up all new passwords and encrpytion, this will knock anyone else off that's playing games with you also. (if they are cloning your MAC Address for instance.)
Read how to properly secure your Wifi here and be careful about who you give the password too.