No AirPort Card Installed

Try the following in order or until resolved:

1. Restart your iMac.
2. Turn the AirPort Off, then On via the AirPort icon in the top menu bar.
3. Go to System Preferences > Network. Verify that AirPort (or Wi-Fi) is not listed as Inactive. If it is, click on the small gear symbol at the bottom of the list of devices to re-activate it.
4. Perform a PRAM reset. Shutdown your iMac. Then power-up the iMac while holding down the <command> <P> and <R> keys simultaneously. Hold them down until you hear the startup chime 3x.