Could be a coincidence that this lack of illumination happened after the security update. Sounds like a ribbon cable or screen backlight inverter failure to me - esp as you can actually see the desktop faintly by 'frontlight'.
But then again... sleep mode - why should it do that?
IF it's the cable, you could try a little hinge wiggling - open the screen wide, then almost shut it, repeat a few times. I recently repaired a Lenovo which would only work if the screen was only open ten degrees. Difficult to compute with that... tho the owner sometimes managed for a couple of weeks.
How do you fancy taking off the bezel & checking for any obvious break or bad connections?
Good luck!