Airport Express suddenly no longer works

Question

I'm thinking from reading the boards that it may have something to do with the 6.3 update, but in case anyone else has any suggestions, I wanted to outline my new problem.
Computer worked (wirelessly) absolutely fine this morning (G5 imac, Airport Express).
Got home tonight - internet connection was no longer working.
I ran my own diagnostic - the airport and the computer are talking, green light on the airport, network showing up in the drop down menu. This means it's a cable problem, right?
No. All of the cable modem lights are still working, but just in case, I unplug everything to reset.
I reset, still having the same problems.
I spend an hour (2 phone calls) to cable company, who resets my modem, yells at me for using wifi (they refuse to provide support to anyone who uses a router), but points out that if I plug the modem directly into the computer, I'll know whether it's actually the internet connection.
So I do this (annoyingly, I might add, since the wifi has previously enabled me to have my modem in the living room near the stereo, and not in my office). After stringing wires everywhere, voila! I have internet (as evidenced by this interminably long post). However, when I reset everything again to use the Express, I'm back to the same problem. All the lights on every device say it's a go, but nothing (e-mail, internet, etc.) loads. When I run the diagnostic presented in safari, all lights are green except second to last, "Internet". Gee thanks for that obviousness.
Cable company won't help me at this point, because as far as they're concerned, when the computer is hooked up "correctly", as in hard line to my computer (as in, not violating their terms of service), it works perfectly fine.
I read a few things here, and try a hard reset on the express. no luck (and yes, I know to reset, repower, etc. all the devices before each attempt). The ONLY thing that I did was yesterday, when I ran all the software updates on my iMac. Doesn't explain why it was working this morning...
I've had this airport for several years. I've set them up in all of parents', friends, etc. homes. I've never had a problem like this (I've had plenty of problems, but they always have fairly obvious symptoms - lights out or yellow or whatnot that hint at something wrong).
Any suggestions would be greatly appreciated

Answer 1

Thanks for the quick response! just to give some further detail...
• Turn off the Airport card and then turn it back on.
-- I did this during the initial attempts (all further attempts will have to wait until tomorrow, as it's past my bedtime)
• "network showing up in the drop down menu" with a check mark next to it?
-- yes, the network has always shown up.
• This thread has a user with a bogus DNS entry.
• Does your Powerbook work?
-- I'll try these tomorrow
• What IP are you getting on your iMac?
-- I was getting the standard 10.10....address created by the airport.
• Create a new Location on your iMac's Network System Preferences and re-create the Airport connection
• Keychain First Aid?
• Temporarily connect via ethernet directly to the Express and check it's configuration again.
• Reset the trusted networks.
-- I'll try all of these tomorrow
Good Luck!
-- Thanks!
iMac G4; PowerBook G4    
iMac G4; PowerBook G4    
iMac G5; PowerBook G4