iMac G5 Can't Connect to Internet over Airport

I have an iMac G5 that has been working fine on my Airport Extreme wireless network for 3 years. Suddenly, I can no longer access the internet over my Airport connection. The computer is on my local Airport network wirelessly (I can access it from my laptop and share files). I can connect to the internet from 4 other computers connected to the same wireless Airport network (2 MacBook Pro's, 1 Powermac G4). In addition, if I connect my iMac to my laptop via a twisted pair ethernet cable and turn on internet sharing on my laptop, my iMac can connect to the internet. All systems are running 10.5.5 and no firewalls in place. I checked my internet preferences and they have not been changed from what they were before when it was working. DOES ANYONE HAVE ANY IDEA WHY SUDDENLY IT STOPPED CONNECTING TO THE INTERNET OVER MY WIRELESS AIRPORT CONNECTION?