I don't know when you put this on the web but I went through the same thing with my toshiba. After taking it to two sources to diagnose it I ended up a geek squade and paid the diagnosis fee. I found out I needed a new motherboard. I had been told I needed a new ribbon cable from the ldc screen to the motherboard and also that I needed a new screen. That was all before I took it to geek squad.
I just got my laptop back and it works fine. It was so frustrating because I could not understand that one day I turned on fine and the next it seemed to start and then went to black screen.
I hope this helps but if you do need the motherboard be prepared it costs almost as much as a new computer. I have a 17" toshiba satellite and could not replace it for the same amount of the repair so I went ahead, you must weight the price of repair to a new computer. :)