I got a Samsung Galaxy S II HD LTE 4G and the screen has stopped working. To be clear, the glass didn't shatter or break, it is just black and no longer comes on. The phone is functioning as I can feel the vibration when I unlock and turn the volume all the way down. The seller has agreed to a refund once I have returned it. However, I don't know how to wipe the data clean. Is there a way to do this via USB? Or should I really be concerned as I am keeping my own sim card and SD card. The phone will have no sim card and no SD when I return it. Any advice would be greatly appreciated.