Mac monitor is displaying screen but I cannot navigate my cursor to it

Solution 1:

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...

Solution 2:

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.