sidebar won't appear on left side

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.

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

If it's system wide I suspect you need to reinstall the OS

I'd quit Sys Prefs & try trashing these & restart...

/Users/YourUserName/Library/Preferences/com.apple.finder.plist

/Users/YourUserName/Library/Preferences/com.apple.dock.plist

/Users/YourUserName/Library/Preferences/com.apple.systempreferences.plist

Change the resolution as a test in Sys Prefs>Displays>Scaled

Are they still in...

/System/Library/LaunchAgents/?

If not reinstall the OS ASAP.

I'm wondering if you actually got to the Users Library?

~ tilde indicates  your user library folder

Apple hid the Users' Library folders...

Method 1:

1. From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2. In the window that opens, enter ~/Library, and click Go.

Yes, should be fine if they're in your Users folder.

I hope you didn't delete the ones outside of your User's folder that the OS uses to replace deleted ones with a default plist.

If you did you'll need to reinstall the OS.

Has it ever been setup with another Monitor on the left?

Corruption can happen for many reasons.