It is a kind of uncommon bug when upgrade to the latest firmware, and you could restore the factory default settings as below. *Please notice that all the personal data will be deleted after restoring the factory default, including the personal files.
Max Series/ Note Series/ Nova Series/ Poke Series:
Switch off the device firstly by long pressing the power button around 10 seconds and keep pressing the "Back" button and plug the device in the power source. (Please do not connect to the computer) When the light shines blue, stop pressing the button. Then the device will go into recovery mode. Press the page button (for Max Series) or the “Back” button (For Note Series/ Nova Series/ Poke Series) to choose "wipe data\factory reset" and press the power button to execute. Then choose "Yes" and press the power button again to restore factory default.
Switch off the device firstly, and then long press back + power buttons, enter into recovery mode, to choose “wipe data\factory reset – yes delete all user data - reboot” to restore factory default.