It can happen if you clicked on view in fullscreen icon
in preview pane previously. So you will find it in your
taskbar as separate icon and main BAS icon will be
just emty gray area. So just click on that new icon
in taskbar to bring it to front view, And click on that
fulscreen icon again to get it back/embed in main
BAS taskbar icon.
However, sometimes in rare cases it is a real glitch,
and you wont find that new taskbar icon, Then just
restart BAS aplication and start your project again.
In most cases you will find that icon in taskbar and
it is due to your previous clicking on fulscreen icon,
which creates separate icon for fullscreen preview,
but doesnt brings it in the front/focus of default
BAS window.