What are you seeing now on the screen? If you still see the screen of the Koreader, it is very likely that there is a hardware defect, as for instance a broken screen.
But maybe it is not broken. In any case you can still try to do a factory reset.
- charge the reader for some time
- hold the power button in the off position for 25 seconds (maybe in your case even longer, since the device seems to be busy)
- release the power button for some seconds
- hold the light button down and switch the device with on
- when the led starts lightening, release the buttons