Aside from trying to reset-to-default-factory-settings that labzuk listed
above I can't think of any other way.
With all of my fiddling around with the reader I've just taken to storing all of my books etc on an SD card, so if I break it I still have everything, and if it gets into a state where it can't read / open a book I can remove the SD card, reboot and it'll go back to the menu.