Why does adb return offline after the device string?
Try the following:
Unplug the usb and plug it back again.
Go to the Settings -> Applications -> Development of your device and uncheck the USB debugging mode and then check it back again.
Restart the adb on your PC. adb kill-server and then adb start-server
Restart your device and try again.
To complete the previous answers, another possible solution is to change the USB socket in which your cable is plugged in.
I had this problem (with the classical answer about using adb kill-server / start-server not working) and it solved it.
Actually, it took some time to find that because Windows was correctly recognizing the device in my first socket. But not ADB. As Windows was recognizing the device, I had no real need to test other USB physical sockets. I should have.
So you can try to plug the cable in all your USB physical sockets directly available on your computer. It did worked for me. Sometimes the USB sockets are not managed the same way by a computer.
Beginning from Android 4.2.2, you must confirm on your device that it is being attached to a trusted computer. It will work with adb version 1.0.31 and above.
adb kill-server
adb start-server
that solved my problem