I would start by reviewing 'Device Manager' and see if the wireless NIC is in an error state (X or !) on the device. If yes go to the properties and check out the error code.
You can also check and update network drivers if any new versions are available from the manufacturer site.
Another good place to start troubleshooting is looking in 'Event Viewer' in the System and Application logs for any errors regarding the device, any services it depends on and/or driver/application errors
If wireless is completely missing from the system tray verify the 'WLAN AutoConfig' service is running.
- Windows key + r
- Type 'services.msc'
- Locate 'WLAN AutoConfig', if the status is blank or otherwise does not say 'Running'