I'm a newbie. When first navigating the Lr screen, I somehow accidentally opened Lr on my second monitor. At the top of the second monitor, the following options show: Grid/Loupe/Compare/Survey/Slideshow.
I'm trying to delete this screen altogether. I've tried everything, with no success.
From the menu bar select Window/Secondary Display. Uncheck Show.
In the lower left corner of the main window you will see two little icons (that look a bit like a monitor). Click on the one that says "2" and that should turn it off.