Hi there!
Just did the same mistake... I think the backlight is broken. Have you tried to look at the screen under a bright light while 'menu' and 'return' buttons are lit?
In my case, just a part of the backlight was off, and screen took some time to wake. Then all the backlight went off, like one or two days later... :( But if I look at the screen closely, I see that pixels are allright, thus my guess at your question.
If anyone knows a way to fix it, I'll be more than happy!