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.."

Wi-Fi ADB (Root / No Root) app for Android devices

ADB - (Android Debug Bridge) for android developers
Test and debug your applications via Wi-Fi connection
Quick access to Wi-Fi hotspots
Indicator of Wi-Fi network connection
Automatic detection of IP-addresses
Pop-up notifications to show of the network status
Quick access to developer tools
Support for Android devices from version Android 2.3.3 and up
Root and No Root Required
No need to use USB cable with root mode
Support Landscape and Portrait screen orientations
"Wi-Fi ADB" is an application that allows your device to be connected to your computer through Wi-Fi connection.

Activate Wi-Fi Mode with Wi-Fi ADB and run on your PC command:
"adb connect device_ip"