Adroid Studio real machine offline

The Adroid Studio real machine appears offline, and the real machine cannot be flashed. For the solution, please refer to the following:

$  cd /Users/cs/Library/Android/sdk/platform-tools/ 

$  ./adb kill-server 

$  ./adb start-server 

$  ./adb devices 

List of devices attached  

0123456789ABCDEF    device