If your box is crashing before you have done anything to it Id be tempted to send it back in case it has a fault that flashing wont cure, but IF you wish to continue & try flashing anyway then on Open ATV as soon as you see the word boot appear on the front panel start pressing the front power button, IF it goes to the boot count then the box has missed the USB:-
reasons the USB can be missed are image not loaded correctly, USB not prepared correctly so box passes it by OR as in your case your box may indeed have a fault