When your Send button changes to a Send Later button Thunderbird has gone offline. Either you told Thunderbird to go offline or your computer lost network connection and Thunderbird changed states due to that.

Next time instead of clicking a bunch of things that you cannot remember and put them back, look in the lower left corner of the Thunderbird window for the Online/Offline icon. It looks like 2 computer monitors. When you have a red x on top of it you are offline. Click this icon to change states.

Now to fix your problem. Open a Write window From the Menu bar select View-Toolbars and click the Composition Toolbar back on. If you managed to remove the menu bar also, press the alt key to make it appear briefly and click it back on when you are in the toolbar menu.

Thank you for your help!

But why does Thunderbird go "offline" when sending a message? It's receiving incoming mail OK, but outgoing messages get marked "Send Later" and then they disappear.

I have the View Menu Bar & Composition Toolbar "ON" (I don't know what this does since I don't see what you see).

This wouldn't be so irritating if the messages didn't disappear for no apparent reason, then I have to write the message again, maybe a day later when the "Send Later" button changes to "Send". By then, of course, all my incoming mail needing replies is stacking up and I can't reply until Thunderbird decides to activate itself again.

Is there an easier way to do this?