Read the statement by Michael Teeuw here.
Black screen instead of desktop after installing PIR sensor
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 @09succotash can u ssh into the machine? is MM running on pm2? do pm2 status, and if not stopped do pm2 stop for each line (use number or name) 
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 @sdetweil I cannot ssh into it. I am able to access the terminal though. MM is running on pm2, I just setup an autostart script for it. I don’t think MM is causing the issue, because the issue started just when I was setting up the PIR sensor. 
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 @09succotash but it can’t hurt to stop it… if you can get to terminal, then its not a desktop problem… 
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 @sdetweil I could only get to terminal, by right clicking somewhere on the screen. I tried stopping MM using pm2, didn’t have any effect. 
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 @09succotash said in Black screen instead of desktop after installing PIR sensor: by right clicking somewhere on the screen ok, so desktop is running, even tho u might not be able to see it… 
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 @sdetweil Exactly. It seems to be running, but my wallpaper, taskbar, and icons are gone. Black screen shows in its place. 
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 @09succotash this is stretch or??? lsb_release -a in the terminal window 
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 @sdetweil This is Raspbian Stretch, yep. 
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 @09succotash if u right click, does the popup menu have desktop preferences? 
 is should
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 @sdetweil This is what mine looks like. The white bar at the top of the screen is vnc viewer, not the screen. I’m using vnc viewer on my laptop. 
  
