Map Windows can be opened in two ways. If it is displayed/visible in the Project Explorer Window and grayed out, right-click and select 'Open Map' or simply double click on it.
Additionally, any Map Windows that have been hidden and not visible in the Project Explorer can be unhidden. To open a hidden Map Window, make the appropriate selection from the Session's context menu as shown below. Note that if there are no hidden Map Windows that can be opened this option will be grayed out.
Select the appropriate Map Window and select 'OK' to open the map or Cancel to quit.