I have 2 imac connected to a Cisco WRT 120N router. One day, the 27" iMac cannot detect the network through the airport while other iMac works normally. The apple system profiler shows the card installed but no network available. I tried to turn it on but failed, I tried to add the network manually but failed either.
Make sure the router does not have MAC address filtering turned on. It may only accept connections from specific computers. MAC is a hexadecimal address known as Media Access Control that all network devices have that is unique to each device. Has nothing to do with Macintosh.