title: adb devices is returning an empty list
body: <p>I have been using my android sdk for more than two months on MAC. Suddenly today my system is giving an empty list for adb devices (for the same set of devices which used to be shown with results previously). </p>
  
  <ol>
  <li>USB debugging options is on</li>
  <li>Killed and started adb server many times</li>
  <li>adb get-state is returning unknown.</li>
  </ol>
  
  <p>Can somebody help me on what to do make my adb devices available and make it work?</p>