Can't Join Wi-Fi Network

I can't access the wi-fi network on my MacBook Air. The wi-fi works fine. I'm using it now on another Mac to post this question but I keep getting this error on my MacBook > "The Wi-Fi network "Hawk" could not be joined. "Hawk" is the same network I'm using to post this question.

I've tried rebooting the MacBook several times but it doesn't help. System Preferences > Network says "Wi-Fi is turned on but is not connected to a network".

Any ideas? Is there a diagnostic I can run to see if there is a hardware problem?