Does the screen light up at all (e.g. starts with a splash screen but goes black when it boots up) or does it never light up even slightly?
What has happened to the device recently, e.g. a software update just before this happened, the phone being dropped,...? Without more information we can't even say whether this is a software or a hardware problem, which makes suggesting a fix impossible. Also, has the device ever got wet (not just recently, but at all)?
I'm assuming that it does boot up, just the display remains black, but can you confirm that?