If a menu option is grayed out
Once a window has been opened from the View menu, that menu option is grayed out which means it is disabled. The View menu shoes an example of this with 'Open the Vehicle List' grayed out.
But don't despair. The active open windows are all listed at the bottom of the View menu and you can click on them there or use the CTRL-F6 jump feature. On the active windows list, the window with focus has a check mark next to it as a visual reference. Notice 'Vehicle on file' at the bottom of the View menu as an example of this.
Once one of these active windows is closed, then the option to open it is again available on the View menu.