Nautilus does not show the left bar, how do I turn it back on?

Just press F9 when you open nautilus.

Another way to do this is

Click on 'view' then check the 'side pane' option.

You can activate or deactivate the sidebar simply by pressing F9. You can also go up to the global menu, then select View → Sidebar → Show sidebar.

Just press F9 or if you like it the 'mouse' way, jut select 'Side Pane' from the 'View' menu.