I´ve experienced similar patterns during freezes and drained batteries, however, most of the time the last picture was still faintly recognisable.
I assume it happens when the device dies down during screen flash. Thus, the pattern should look differently on that occasions with screen flash turned of.
If so, the substrate might not be broken at all for these two - it could still be a "trivial" crash - thus the pattern - which damaged something else preventing the CyBook from starting and / or accessing the screen. This does not neccessarily mean the screen is not dead anyway, I am afraid.