One question for clarification's sake...
When you say the screen is black and it won't boot, what EXACTLY is the situation?
A) No power, no fans, no lights, no nothing?
B) Power comes on, fan spins, but otherwise there is no activity?
C) Screen comes on, some post information is displayed, perhaps an error message?
The last one is added only because I have had people tell me "It doesn't come on" then they tell me after that oh yes the screen is "black" and displays an error message.
If it's the FIRST one, odds are good you've got a dead motherboard (or a bad battery AND power supply, but much more likely the board).
If it's the SECOND one: A hard reset as Saugen suggests might work, or swapping out memory and/or hard drives may work as QHN suggests.
The third, if it's the case, would depend on what error would be displayed.
Either way, let us know what the situation might be!
O Shadow Fox O
Edit: Okay, that's technically three questions, but still...