Missing Photos tab on side bar

I finally managed to reproduce the problem. Check, if your Mac has accidentally been started up in Safe Mode. This can make the "Photos" view vanish from the sidebar.

See: Photos on Catalina: If the Photos View vanishes from the sidebar, check if the Mac is in Safe Mode - Apple Community

On my Mac the "Photos" View vanishe from the Library section of the sidebars, whenever I restart the MacBook Pro (13-inch, 2017) in Safe Boot Mode. Restarting the Mac in normal mode will bring the Photos View back.

If you restart the Mac and are seeing a red message about "Safe boot" on the login screen, the Mac has started in safe mode (Use safe mode to isolate issues with your Mac - Apple Support).

And when I then open my Photos Library, the Photos album is missing.

Restarting in Normal Mode will bring the Photos album back.

Sometimes it can happen, that a Mac is starting accidentally in Safe Mode, without us being aware of it. I am not sure, if it still possible in Catalina, but it can happen, if the Mac is starting up, before a bluetooth keyboard has been turned on.

If you're not sure that your Mac is started in safe mode, you can use System Information to check. The Software section of System Information lists Boot Mode as "Safe" instead of "Normal" when you're started in safe mode. (https://support.apple.com/en-us/HT201262)

That's strange: it seems like the main library is hidden. Ctrl - 1 shortcut should lead to the whole library, Ctrl - 2 to memories and so on (following the commands available in View -> Library).

I'd try to repair the library as described here (make a library backup first!):

https://support.apple.com/en-us/HT205966

Many users have reported this problem. I have not yet seen a fix so far.

If you select a photo in "Imports" and ctrl-click it - is the option "Show in all Photos" available in the menu? Does this command work?

If you press Ctrl – 1, do you get the whole photos library?

What Safe Mode does among others, is disabling accelerated graphics or custom fonts.

If your Mac is not in Safe Mode, but still the Photos item is missing, it might point to the graphics card or the other items in the list of items disabled in Safe Mode:

• ....
• Loads only required kernel extensions
• Prevents startup items and login items from opening automatically
• Disables user-installed fonts 
• Deletes font caches, kernel cache, and other system cache files

Some features of your Mac aren't available when it's started in safe mode:

• You can't play movies in DVD Player.
• You can't capture video in iMovie and some other video apps.
• Some audio input or output devices might not work.
• Some USB, FireWire, and Thunderbolt devices might not be available.
• Wi-Fi networking might be limited or unavailable, depending on the Mac and macOS version you're using.
• Accessibility features like VoiceOver might not work as expected.
• File sharing is disabled.

Safe mode in OS X Leopard v10.5 or later also disables accelerated graphics. This makes the menu bar appear solid even if "Translucent Menu Bar" or "Reduce Transparency" is selected in System Preferences. Your screen might also "blink" or "tear" during login when your Mac is started in safe mode.

But I don't get the side bar

It shows the complete library but not the side bar

Interesting. When I'm home I will test this right away. Great!