Read the statement by Michael Teeuw here.
Black screen instead of desktop after installing PIR sensor
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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? 
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 @09succotash I might try to change desktop with login 
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 @sdetweil Been there, done that, didn’t work. I’ve been searching Google for this issue, and came up short. Didn’t find much results, but I will try some more searching, 
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 I had a similar problem when I used MMM-PIR-Sensor module. Everything had been working for about 6 months then I updated something and it stoped working. I re-installed everything several times and through a process of elimination and google searches it acmes down to this particular module. I have now got it a working again by finding the latest update from a branched version of MMM-PIR-Sensor module. I never did find the cause. 
