All of a sudden I got an error from Gnome (don’t know what it was) and the Application Launcher button (not sure how to call it - my desktop is in French - but I mean the bottom left button from where to launch applications) disappeared.
Restarting the computer didn’t make it back.
How can I restore it ?
I can see that right-clicking on desktop allows me to Create a launcher. But I don’t want to create a new one, just to get back the original one.
Right-Click on the panel >> Add to the panel
I can see:
If I select the second, and click on Add, I’m offered a list of application groups to select… So it seems it’s for creating a new application launcher again.
Is there another way to get the original Launcher back ?
Got it back ! - Phew (felt like naked without it, even with the great rescue help of Alt-F2) 
It was not an Application Launcher I had to restore, but to add “Main Menu” to the panel (bottom in the list).
Right-click on Panel >> Add to the panel >> Main menu