Well, I've just experienced this... happily reading a book, well into the wee hours of the morning. About maybe 150 pages back it was at 40% battery, and all of a sudden the Cybook "breaks" nothing works. Tried the reset etc. A few minutes of panic later I plug in the USB cable, and the LED goes red, I press the power button and it resets the screen and powers on
I've never given it a proper charge of the battery, so I'm guessing that's what the issue is.