AirPort from a MacBook

Some weeks ago my airport started showing the Message: "there was an error while joining the network". The connection still worked, some weeks later that message lead to not connection at all. My wlan-router seems to be OK and now i can only connect over cable... can it be something from the mac? what can i do?
ART

I have a similar problem, My cable modem works plugged into my mac, but not into the airport extreme. The Airport say it has no IP address, OR, it shows connected, but can't resolve dns requests. I tried to set it up manually, but no dice either.

Any ideas?

I tried changing the settings from the router, the only way to use the wlan is when i open it to everyone, so it must be a problem with the passwords. Changed to WP-2 to WEB etc... nothing works... any tips people?
ART