Error in connecting Samsung A3 to Ubuntu 14 (USB); adb unauthorized (no pop up on device too)
You might be able to clear the list of authorized devices so the authorization list is reset to its default if it is in some kind of corrupted state right now. You can do this under Settings > Developer options > Revoke USB debugging authorizations.

Another thing that is frequently overlooked is rebooting: rebooting both your Android device and your computer might make it work. Another thing to try is connecting the device to your computer and then disabling and re-enabling USB debugging, my personal experience is that this fixes a lot of ADB problems.

Users on stackoverflow reported that switching the device to camera mode might work as well.

If all else failes, you can also try to manually authorize yourself on the device, using Flavios answer on Stackoverflow:


Move the authorized key file: mv /data/misc/adb/adb_keys /sdcard/adb_keys_backup (you can also delete it if you want)
Send your adb public key to the device (look in the ~/.android directory)
Stop adbd using the command stop adbd as root
Copy your key to the authorized devices list: cat adbkey.pub &gt;&gt; /data/misch/adb/adb_keys
Start the ADB daemon again: start adbd (as root)