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It is possible that the screen has broken, rather than it being a firmware or other problem. If it broke while the sleep screen was showing then it will continue to show that screen even if the rest of the device is functioning normally.
To test this, check if the LED flashes its normal sequence when you turn it on. Then try connecting it to a computer via USB (you will need to tap the screen where the connect button normally shows, even though you can't see it.) If it connects so that you can see the device's filesystem from the computer, then it is probably a broken screen.
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