|It might depend on what you're hitting but usually you'd open control panel and then 'power options' and change the 'turn off monitor' option to something more realistic.|
Or maybe you have some screensaver that's blanking it. Open 'display' in control panel, click the 'screensaver' tab and make the adjustments there.
If it's neither of those then there may be a physical problem with your monitor or video card.
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