Ever switched on your laptop and thought; Hey there's nothing coming on the screen, only to notice as you move it towards you for a better look, the light catches the screen at a different angle and there's a very faint/dark image displayed? In every laptop lcd screen there's a florescent tube that illuminates the screen from the back (hence the term backlight) if this fails or the mechanism that starts it up fails then your display will become dark/dim to the point of being unreadable.