Many reasons.
Screensaver
Hibernation
Wrong Resolution
Wrong Input (Digital / Analogue)
No Signal (Check Cables)
Screen not turned on (It happens)
Graphics Card Fault
Screen Fault
If your computer screen goes black, the first thing to try is to turn your computer off, and then turn it on again. This allows the computer to re-boot.
If it is just the monitor that is shutting off, then there might be a problem with the monitor with staying on. If by 'screen', you mean the actual computer goes off and produces a black screen, then your computer might be set to go into hibernation quickly, or it might have a mode where it goes into a blank screen saver quickly. Check the computer's settings to fix that problem.
Sounds like the screen saver is coming on and messing things up for your game. Turn off the screen saver and see if that helps.
As in turn it off, or change when sleep turns on? Right click desktop > Personalize > Screen Saver > Change power settings You can set how long before your computer goes to sleep, or turn it off altogether.
Push from the back of the screen.
A computer can not turn off by itself. The monitor just turns blank it you leave it. To turn off the computer you have to prees the on button on your computer. If you press the botton on the monitor it is not turning the computer off... just the screen.
If your computer's splash screen can be modified (turned off or on) this setting is going to be found in the BIOS. You will have to consult information for your specific manufactuer to see if the splash screen can be turned off or on. Most can be, a few cannot.
pixel
They are actually looking at a large computer screen off to the side, which turns out to be right in front of them, but off-camera. This screen shows the part added to the green screen.
use your mouse
Turn the computer off then turn the computer back on
It goes off screen, and never returns.