Read the statement by Michael Teeuw here.
Black screen instead of desktop after installing PIR sensor
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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. 
  
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 @09succotash ok, select Desktops or reconfigure 
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 @sdetweil Clicked reconfigure, nothing shows up. Clicked desktop, just shows a list of desktops. Shows 4 desktops. I’ve been through them all, shows the same black screen. 
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 @09succotash in the terminal window, if u run sudo raspi-config 
 and look at boot options…beyond that there are 1 few (hundred) posts about problems like this if u search google 
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 @sdetweil Boot options, and then what? 
