What you show is the 'Connections' panel.
It should be listed under the 'View' menu - so post an image showing the View menu list of menu items.
Whenever the UI is not rendering normally, the first thing to try is (from the main application menu bar)...
Windows > Reset Windows To Factory Settings
If that does not work, try restarting the application after...
Tools > Features > Clear Cache
If that does not work, there must be corruption in your user settings for the specific version of SQL Developer, so you can try renaming or deleting that folder.... %APPDATA%/SQL Developer/system184.108.40.2069.0537