This problem occurs when you connect a computer directly to a cable modem. Whenever you do that, the cable modem will refuse to communicate with anything else until you power it down and allow it to reset. Power it down, give it a few moments to forget what was connected to it, then connect the modem to your Extreme, and power it back up again. Everything should be back to normal.
Make it a habit to perform this power down exercise every time you change what's connected to your modem.
Cable companies request that you connect a computer directly to the modem because it makes troubleshooting easier for them. As far as they're concerned, if their modem is working, their job is finished.