My dad's nexus 7 no longer boots. After the boot cycle starts and the google logo comes up, a big green outlined arrow with the word START in it appears, that points to one of the buttons. Nothing else happens. If you press the button it points to, it flicks back to the start box. If you hold down the power button it reboots back to the START. What is this thing and how do I make it go back to rebooting?