When start up the screen is blank, not...

Press the F2 key several times to make sure that the screen brightness is up.
Put your ear down on the laptop on the zone left of the trackpad and below the keyboard and listen for the little whirring and clicking noises of the hard drive. If it is making noises, just walk away for at least 30 minutes and see what happens in that interval.
Look carefully at the screen in good light and make certain that here is no image on it. You could just have a bad backlight.
If so, you can connect the DVI port on the MacBook to a computer monitor or most flatscreen TVs and regain access to the screen until you get the backlight repaired.
You might try that regardless. Even if your display is totally inoperative, the DVI port may still function.
If the drive has stopped making noise and the screen is totally blank, 1. Try starting up the MacBook with the DVD that came with it, and if that doesn't yield anything, 2. Take the MacBook to the Apple Store and ask for a Genius appointment.