My experience is from Visual Studio Code and not Visual Studio 2019 but I suspect they are most likely similar..
Press F1 and type "View: Toggle Activity Bar Visibility" and click on it when it shows up.  That should make your activity bar visible again.
I am in the Microsoft Visual Studio environment, most screenshots of this environment show an activity bar in the left sidebar. However, I cannot get this to appear in my display, I have sought to access this activity bar in view but cannot see any reasonable options.
Any thoughts?
visual studio Display, how to access activity bar?