The sidebar of finder shows nothing except iCloud Drive, Network location and Tags

iMac. MacOS Mojave

The sidebar of my finder window suddenly shows nothing except iCloud Drive, Network location and Tags. Documents, Desktop, Home etc have disappeared, even though Finder Preferences still indicates that they should be shown. How do I fix this?

Furthermore, I cannot drag the Home, Documents, Airdrop etc icons onto the side bar to create new aliases

​[Re-Titled by Moderator]