Have you checked the driver(s) for your monitor? Sometimes the OS will automatically install a generic driver rather than a driver specific to the device as provided by the device manufacturer; check to see, also, that you have the latest drivers for your specific OS version; Win 10 drivers very often won't work with earlier OSs and vice versa...
Also, as an afterthought, you might want to check the display settings to see if your display sleep or screensaver setting may have been altered...
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