Have you tried a hard reset yet?
Do a hard reset, wait, see whether it gives a message, similar to 'firmware (or system) update fail'? If so, update with Sony's original firmware.
Power off (press powerbutton 3 seconds and choose yes) and back on. While the ereader starts it shows "opening book", at that moment press -and hold- the buttons next page and back (button 2 and 4). Allow the Home screen to fully load (Loading... should no longer flash in the top left corner of your screen) and then, while you still pressed the next page and back button... release the back button, press the back button, release the back button (while still pressing the next page button), and at last, release the next page button. Then the screen shows: Would you like to execute Factory Initialize? Then choose "yes"."