There are quite a number of reasons that may lead to the black screen on a computer. The reason vary so highly depending on whether its a desktop or laptop.
1. For a desktop, before you may freak out make sure to check that all the cords are well connected because the black screen may be as a reesult of loose codes
2. Make sure that your computer boots, that is to say, make sure that your computer turns all the way on when you actually switch it on. When you push the power button listen to the computer and check that all the lights are actually lighting up. Because when this doesnt happen it may be the cause of the black screen. If this is happening rushing the computer to the rrepair be the best idea
3. If your using a desktop try using another monitor instead, just in case the problem is with the monitor
4. Try reinstalling the disk if you are not sure how to go about it make sure you see an expert to do it for you because sometimes the blackscreen may as a result of a problem with the operating system.