Indeed, a calibre question belongs in the calibre subforum. Otherwise our calibre experts might not see it.
There is a button in the bottom right corner for each of the four different view options.
Toggling on/off:
the book details panel
the book list vs. cover grid
the book covers carousel
the tag browser (what you are missing)
Click the tag icon to toggle the tag browser back into view.
Edit: Aughhh! Ninja post! ^^