I'm using a VGA adapter to connect my Macbook Pro to an external monitor and I see the Macbook screen on my external display but I'm not able to move my cursor to it. It was working just fine a day ago, all of a sudden my cursor is not able to navigate there and I'm not able to drag any open windows to the screen either.
Check that the virtual display arrangement in System Prefs > Display > Arrangement is the same as their physical placement.
You can only move your mouse across between the two at the side where they join.
You can drag & drop either of the virtual screens until they match your physical layout. When dragging, both the virtual & real screen will display a bright red border, so you know which you are working with.
The white menu bar can be dragged separately & should be placed [if it isn't already] on your main monitor.
Also note that if the screens are not the same resolution [size actually doesn't matter, only resolution, as displayed in that control panel] then you may have a smaller 'gap' where you can pass from one to the other, as in this picture...
It turns out that it was inverted. Instead of navigating to the monitor from the left side of the screen, I should have been going through the right side. My monitor was set up to the left of my computer so it was a bit disorienting so I changed it.