Try a hard reset using Recovery Mode. NOTE: THIS WILL DELETE ALL DATA.
Turn off your device by pressing Power button.
Hold down Volume Down + Power key.
As soon as menu appears, release all the buttons.
Navigate to "wipe data/factory reset"Reset with Volume Up and Volume Down keys. Use Power button to select.
Choose option "Yes--delete all user data".
To reboot select "reboot system now".
You have just completed hard reset process.