I attempted to decrease the screen resolution by way of the System Settings, and when I clicked Apply, the Application Menu stopped opening on the panel. Hovering over the KDE icon, the help tag opens and tells me this is the Application Menu and clicking on the KDE icon a thin line appears immediately above the icon. Even though I put the screen settings back to default, the Application Menu still will not open the menu.

It's as if the Application Menu is opening, but not displaying for some reason.

If anyone knows how to fix this, I would appreciate the help!

Which exact Plasma version and distribution is this about?
Did you log out and log back in again?