Users often find that they have accidentally dragged something from the left side of their Finder windows. Sometimes it's Network, sometimes it's Hard Disks, sometimes Desktop or one of the other items. They can't always figure out how to get it back, it's not under the View options as you might expect. Some users never even remember dragging it out of the Finder window.

click on your desktop to pull finder foreground

go up top and under Finder, select Preferecens.

Click on the Sidebar option at the top of the window.

Check anything you want to show up in your sidebar.