Power settings etc all set to NEVER. 
This is my office build, W10 all updated. Started doing this about a month ago. 
The screen goes blank after about a minute if you don't move or touch anything. 
It's still on, it's glowing dark. My other desktop machine goes black when it's asleep. 
 
I changed monitors.
Pulled my Video card and using on board for now. 
Set every power setting I could find to Never sleep. 
Could this be hard drive related? 
The OS drive is a Blue WD. 7200 RPM 1 TB. There are 3 other regular drives as well 
750 watt PS 
Intel Pentium G3220 Dual Core 3.0GHZ Processor LGA1150 Haswell 3MB Cache  
Avexir Core Series 8GB 2X4GB DDR3-1600 11-11-11-28 1.5V White LED  
ASRock H97 PRO4 LGA1150 3.0X16 3XPCIEX1 2XPCI Motherboard  

Checked Windows Event Viewer and see what's there.
Thermal problems can cause issue that you describe.

Good idea, Well I looked and there are a lot of random errors today,, but then I've forced shutdown a few times as I have been disconnecting HD and video card etc. 
The monitor display blanking out happened over and over and I see nothing in the last hour.  
 
Thermal ? . I built this box and It's well cooled CPU sitting at 26F right now. But worth looking into. 
My other suspect is a failing Power supply. It's a Cooler Master EX2 750, I had trouble before with Cooler Master.  
 

When the screen goes blank, what happens when you tap the power-button?
Don't hold it in 4 seconds... just tap it.
If it shuts down gracefully, then the machine was still functioning inside Windows (though you can't see it).
 

Do you have two monitors set up? There are some reports of this problem occurring if the Windows second monitor settings have the second monitor option enabled when no monitor is actually connected.

My screen save was set to 1 minute - blank  
This was ineradicable dumb!! I posted on the NCIX computer forum and first answer was right. Check your screen saver! Duh! Boy do I feel dumb. 
But honestly, I haven't use a screen saver for ever. I guess this came about because I got bored one day and downloaded a custom theme! Obviously a way to mess things up. I was right there to shut the stupid sounds off and didn't notice the lack of a red cross on the screen saver icon. 
Here I am digging deep inside the control panel when it's a dummy setting under personalise. 
Any way's thanks for the great tips.  
Now I can put the video card back in, the on board video doesn't seem to run 2 monitors properly.