[android-studio] Android Studio - Device is connected but 'offline'

Earlier for almost 3hrs I did:

  1. I tried everything given in several sites and my android device never came online.
  2. when I was running adb kill-server and then adb-startserver the "Device File Explorer" on the right bottom of the android studio showed "Device is not online (DISCONNECTED)".

Here is how solve this:

  1. Revoked all USB debugging authorization on the device under "Develop options"
  2. And I added sudo to command "sudo adb kill-server" and then " sudo adb start-server".
  3. After this the message in "Device File Explorer" in android studio changed to "device is pending authentication please accept debugging session on the device". But no message appeared on the device. Tried stopping-restarting adb, connect reconnet usb cable, stop-start usb debugging but nothing worked.
  4. Went back to device and changed the device usb settings from usb charging to "PTP", and, restarted the Android studio. And, boom, the message appeared on the phone to accept the debugging session on device.