My first sign of a problem was a greyed-out Send button in the File menu of the Write window; only the Send Later option remained. In trying to fix this, I deleted something, resulting in loss of both the menu bar and button bar.
The Thunderbird version is 11.0.1.
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CHAMP0Go to View > Toolbars and 'check' the Composition Toolbar back...
(If the toolbar with View and so on is missing, Hit F10 to have it show up temporarily.)
The Menu bar is the one with File, Edit, View, Insert, Options and so on.
The Composition Toolbar is the one with the icons on it, 'Send', 'Spelling', 'Attach', and so on.
You can turn the toolbars on and off and add or remove icons from them in both the main program and the Compose window the same way.
'Send' changes to 'Send Later' when Thunderbird is off line - not necessarily when the computer is off line, but when Thunderbird is. The icon of two computers in the bottom corner gets an x on them when it's off line. Float your mouse over it and a tooltip will say if it's on or off line. Toggle it on and off by clicking on that icon.
If you don't see it there, go to View > Toolbars and make sure the Status Bar is 'checked' on.
If Thunderbird won't go back on line, then your computer is off line and you have to hook it back up to the tubes.