"Your computer isn't connected to the internet"

I hope I can make this succinct....

Earlier today, Airport suddenly quit working. I couldn't access the internet (getting message "your computer isn't connected to the internet") and my email wouldn't work. I restarted the computer and it worked fine, for awhile. Then the same problem arose.

I ran the network diagnostics. When i started, all Network Status lights were red (Airport, Airport settings, Network settings, ISP, Internet, Server all said "failed".)

I selected the location "automatic" and then chose network port configuration "Airport". That made the airport light turn green, but the rest remained red.

It asked me which airport network to join and i picked the appropriate one. I know the network is working because I'm using it on a separate computer to type and send this. The airport light remained green and the rest remained red.

It asked if I use PPPoE or DHCP. Since I've always used DHCP in the past, I presume that continues to be the case. So I chose DHCP.

It asked if I use a cable modem or DSL. I said yes, but it didn't need to be reset.

The first couple times I tried this diagnostic, the ISP button would turn yellow in the beginning and later a pop up would showup saying "yor network settings have changed" or something to that effect.

Any idea what might be causing this and/or how to solve it? I presume a network setting just got messed up but I have no idea what it might be.