Even though "Bonjour computers", "External Disks" and "Connected servers" are checked in Finder Preferences->Sidebar, they do not appear in the sidebar.
Start with these...
Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes.
Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.
Have you done a PRAM reset, CMD+Option+p+r...
http://support.apple.com/kb/HT1379
In fact, do 3 in a row, takes a bit of time.
Intel-based Macs: Resetting the System Management Controller (SMC)...
I force restarted the finder and they re-appeared.
(Cmd-Option-Escape), then select Finder and Relaunch.
Also survived restart. So strange, this had been happening for months after daily restarts.
"Network" and "External disks" not appear on MacPro (trashcan) after upgrade to High Sierra and Catalina
