VSCode SCM tab suddenly disappeared

As @rioV8 commented - what solved it for me was to right click on the icon and click "Reset Location"

I had to use the View: Reset View Locations command to solve this problem in Visual Studio Code version 1.47 (macOS - Catalina).

To do this, just press Cmd+Shift+P (macOS) or Ctrl+Shift+P (Linux/Windows), type workbench.action.resetViewLocations, then press Enter.