With a MacBook Pro 2019 16-inch running Monterey 12.2, suddenly in the Finder sidebar almost all of my Favorites have disappeared. Now under Favorites, besides two folders I had originally dragged there long ago, there is only Dropbox, Desktop and my User folder icon.
imo, this is a long-standing Mac OS bug (or feature? ha), from at least Sierra, maybe earlier. For me, the Finder sidebar favorites are mostly shared folders on my local network, and the trigger to them vanishing is a server reboot or other glitch in connectivity on my LAN. Once these favorites vanish, the only solution I’ve found is to re-add them back to my Finder sidebar(s).