adb return offline

The status of devices always are 'offline' in adb like below

------------------------------------------------------------------

List of devices attached 

XXXXXXXXXXXXXX offline

------------------------------------------------------------------

There are something to try to fix it.

1. reboot devices and PC.

2. do.

adb kill-server

adb start-server

3. If your adb's version is 1.0.29, update it. over 1.0.31