how do you fix a black screen?

my friend has a new compaq desk top computer, that's all i really know about it, but he was at home and left the room for a minute or two and came back to find his daughter standing on the keyboard. now it doesn't do anything. just has a black screen. he knows to check to see if it's plugged in and all that. what could be the problem and how to fix it? i'll try to get more info from him and add to this.