Had exactly this problem - couldn’t find an answer so contacted Amazon support who were brilliant. In short, turn off, hold start button and volume down button for a couple of seconds til machine starts, this enters the “techy” menu. Use volume buttons and then power button to select “wipe data / restore factory settings” option. Then repeat off, back to techy menu and reboot. Takes 5-10 minutes to then restart and you have to go through usual set up stuff again . . . .Amazon can talk you through this . . . . .