My Desktop folder is vanished in the left bar of Nautilus, but I can still open it using the right click in the desktop and selecting "Show the desktop in file" or choosing it from the home folder in Nautilus. I'm using Ubuntu 19.04 (Disco Dingo).
Go to Files or Home to open a Nautilus window and click on
Files on the top bar of the screen alongside Activities.
This will give you a drop down menu from which you can select to invoke the Sidebar (or deactivate it if already selected).
Then with a ✔ alongside the option as required the Sidebar will be reactivated/recycled.
Desktop folder is still not shown in the sidebar it is probably because the
Local Network Share has been lost.
It can be restored by opening your
Home folder, right click on the
Desktop folder icon and select
Local Network Share from the drop down options.
/home/YOUR-USERNAME/Desktop in Nautilus, then press Ctrl + B to bookmark the location, and you'll have a "Desktop" item in the Nautilus sidebar.