Updated June 6th, 2019.
It’s very handy to have some shortcuts in the left-hand pane (the “sidebar”) of your Finder windows. In the screenshot below you can see I have shortcuts, in the sidebar, for a bunch of folders. One click on a folder in the sidebar and it opens in the main window pane to the right, even though I don’t know where the folders really are.
Sometimes the list of Favorites disappears, and it looks like this:
You are probably guessing that the Favorites are still there, but hiding. Good guess!
Try moving the cursor until it is parallel with the word “Favorites.” See the tiny, grey-on-grey writing that says “Show?” Click it, and your stuff isn’t hidden anymore. Of course you can hide things again later.