If you don't see the Table of Contents (TOC), it is probably 'collapsed' at the left side of your screen (if viewing on a computer or tablet).
On smaller screens, or to create more viewable area, the TOC can be temporarily hidden/collapsed.
To re-enable/open the TOC, just click on the double-arrows (usually in the upper-left) of your view. This will slide the TOC back on screen.