Laptops have these weird commands that are usually colored blue, or maybe red on the keyboard (usually under one of the Function keys at the top of the laptop keyboard). One of them toggles the external monitor as some laptops wont "see" it until you order it to look for it.
Find the function key [fn], hold it down and press a button that looks like a box/TV/box with LCD written inside of it. Like these below:
It varies based on model. Could be Fn + F5 or F6 or whatever. You will have to do it once , twice or maybe 3 times to cycle through the options.
Doh! I thought you were talking about an external monitor!
In this case, the graphic settings (screen resolution) may be set to a level so high that the laptop cannot display, so it appears black.
You can try hooking an external monitor to it and see if it will output to it. At least that means that the computer is still usable.
Hopefully it isn’t a dead LCD.