Windows 7 boots to a BLACK screen with no cursor, unable to use f8 function?

Im using a toshiba satellite L505D ES5024 and when I turn the computer on, it would boot to the toshiba screen, and then a blinking underscore would appear on the top left corner and dissapear. The screen will then remain blank as I have left it like this for an hour already.. I have no windows 7 disc, and the f8 function does NOT work. it only leads to the same blank screen. I would appreciate any help. Thank you