This has to do with an issue of hooking up a second monitor to your system, but no picture showing on the screen on one of your monitors.
I’ve had this problem when setting up dual monitors before. I unplugged my second monitor and when I plugged it back in, the screen remained black.
The solution is simple. Right-click on your desktop, and click on Screen resolution.
Usually, this clears up the problem.
Another thing, at one time my monitor started displaying an odd resolution. Everything looked extra big on the screen. The resolutions were all set and adjusting them had no effect.
Again, clicking on this Detect button solved that problem as well.
The Identify button just displays a big white number on your monitors. Number 1 for the first, number 2 for the second, and so on.