"File Open" dialog is missing sidebar items
Go to your user library in Finder. Hold down ⌥ while opening the Go menu and click Library. 
Navigate to the Preferences folder.
Remove any files that are or contain com.apple.finder.plist. (The removal of those files will very likely reset your Favourites list in Finder.)
Restart or log out and log back in again then empty the trash and try again. 

Restarting might not be necessary. As madpoet says:


  You can also relaunch Finder if you don't want to reboot or log out. Right click on Finder icon while you're pressing ⌥ and you'll see the Relaunch option there.
  
  





Alternatively, you can use this Bash oneliner by Christophe Marois:

cd ~/Library/Preferences &amp;&amp; sudo find com.apple.finder.plist* -exec rm {} \; &amp;&amp; killall Finder