Hi Ash,
I guess you accidently moved the pane to the left.
Try Command-2 to get it back or select the Pages & Resources item in the View menu.
You can adjust the width of the pane somehow (limited range) by right-clicking and dragging the pane to the left or right.
See below.
At a certain point the cursor changes it’s appearance and the pane is gone (see below). Press Command-2 (or select the menu item) again to get it back.
Hope this helps,
Menno.