How to fix adb device offline

Output via USB
~$ adb devices
List of devices attached
0123456789ABCDEF    offline

Output via Wi-Fi
~$ adb devices
List of devices attached
192.168.1.2:5555        offline


a) At first check the adb version, should be 1.0.31 or 32, if not, upgrade to the newest SDK / adb

    Run the following command:
        ~$ adb version
        Android Debug Bridge version 1.0.31

    Run the following command:
    ~$ android update adb

b) Go to "Development options" on your phone and click "Revoke USB debugging authoriz.."

Try to:

1. Unplug the USB cable and plug it back again.
   
    Run the following command:
        ~$ adb devices      
        List of devices attached
        0123456789ABCDEF    device


Try to:

1. Unplug the USB cable
2. Connect and disconnect your phone wia Wi-Fi

    Run the following command:
        ~$ adb connect 192.168.1.52:5555
        ~$ adb disconnect 192.168.1.52:5555

3. Restart the adb on your PC.

    Run the following command:
        ~$ adb kill-server
        ~$ adb start-server

4. Restart your device and plug the USB cable again.

        ~$ adb devices
        List of devices attached
        0123456789ABCDEF    device


Try to:

1. Disable USB debugging on your phone (Device not connected to PC)
2. Re-enable USB debugging on your phone
3. Connect you phone to your PC via USB, you should see the pop-up window.   It will ask you on your phone is you want to allow the connection. Tick always allow.