Internal Airport Extreme-ly annoying

Hi Folks.

I've lost my (internal Airport Extreme) connection to the internet, which was via a Belkin N1 wireless router. to my G4 laptop 1.67ghz. Wireless Card Firmware Version: 405.1 (3.90.34.0.p18).
I get 4 ok lights on the Belkin but the 5th which indicates a wireless connection won't light up. I can connect with an ethernet cable plugged to the router and a PC plugged to it connects ok too. The Mac recognises the network on wireless but I get an airport error message saying I can't connect.
Tried talking to Belkin but seems to be an Airport problem.
Graphic designer, working from home, now making no money! Help.

Did you restart the computer, restart the modem, restart the router, check for airport software updates? You can also delete your network .plist files and re-enter your settings. Can you bring up the Belkin configuration page when the PB is wired to the router? Try restarting the router there, too.
Anyway, those are the things I'd try first.