I have looked for a solution to the problem on Google and still haven't found a solution yet. I have a Toshiba Satellite L500-1DT and a few days ago my screen went black (when i look very closely i can see just a outline of the desktop icons) so i know the laptop is still on. Things i have done already: I connected the laptop to my t.v. using a VGA cord. when i do this the screen appears perfect on the t.v. and when i press fn f5, it usually comes up on my laptop screen as well. But its annoying having to connect up to my t.v. every time. I also looked at my screens brightness and changed from my normal display plan to eco and back again. My computer isnt going into hibernation or anything like that i think because it works fine when connected with the VGA cord.
it sounds like your backlight has possibly gone bad. Unfortunately, it's a very expensive fix since you most likely will need to get a whole new screen to fix it.
also i found this on the internet ... you can try it too
""""TheLabel was close!! I found when I took it apart that the switch that tells the laptop that the screen is "down" was held down by some plastic insulation that I accidentally moved. This made the laptop think the screen was always down. I moved it over and everything works fine now!!"""""
another helpful fix
""" Good news!
-------------------------------------------------------------------------------- Hello TOSHIBA-Users! My advice for your dark screens: Every Toshiba laptop has a small wheel on the right hand side of the monitor which you can pull up or down in order to adjust the screen. You will have accidently touched it and moved the screen to dark. ThereA´s no need for repair!!!!!"""""