It says "No Airport Card Installed" in the status bar.
There are 2 main reasons for this message:
1) The AirPort card has failed and will need to be replaced
2) The AirPort card is not making good electrical contact with the card socket
In either case, this is a job for a service shop, so you should take it to an Apple Store....if you have one in your area....or contact Apple Support directly for instructions.