You probably use the Sidebar in Finder every day. You have quick access to your most commonly used folders and access to the network drives.
What happens when one of your items disappears, most commonly AirDrop.
Let's get AirDrop back. When you are in Finder you can go to your Menubar and select Finder-Preferences.
Then you can check on AirDrop to get AirDrop back in your Finder. See all the other items you can add or remove from your Finder Sidebar. Check out Advanced, General, and Tags to see other ways to customize Finder.