Why does my computer screen occasionally black-out?

For some unknown reason, recently my screen has gone dark at random times during random activities, ranging from just logging on to being on the internet to playing video games. But here’s the weird part: If i look close enough at my screen when this happens, I notice that everything is still there. It’s as if the computer has no idea what in the world is going on with it’s visual output. Who knows why this is happening?