You may have inadvertently hidden some toolbar, in particular maybe View > Toolbars > Composition Toolbar when composing a new message. Check this article to learn more about the view options available in Thunderbird:

https://support.mozilla.org/kb/getting-started-thunderbird-main-window-supernova

Many thanks, I am sure you are right, problem is I don't know how to delete my mistake ... I will try until I find ! Thanks again

help me to place again "envoyer" under the case "fichier" so I can send my messages easily

Your screen shot shows the tool bar for the Thunderbird main window which doesn't habe an item "Envoyer". I you open a compose ("new message") window you will see another tool bar which contains "Envoyer" and/or "Envoyer plus tard"