Operating system version? InDesign CC version?
Does your sidebar show up in other applications when opening or saving?
Mac Os 10.1.11
Indesign CC 2015.2 Release, 18.104.22.168 Build
It seems to vary from app to app, but "Favourites" has disappeared from all the apps except when I'm in Finder itself.
If your Favorites has disappeared from all the apps, then it's not an InDesign issue.
You'll get a better response if you go to www.apple.com and use their forums. Look for the one for El Capitan.
You said Mac OS X 10.1.11. I assume you meant 10.11.1. By the way, there was an update to 10.11.2 yesterday so you should first update your operating system version in the App Store.
I updated my OS and it seems to have resolved itself.