Copy those messages archived accidentally back to their original folder(s). Once copied successfully you can delete them in the Archive folder.

Tks for your message. The issue is that, even though it says the "archived" messages goes to "archived folder at my email, I have no folder with such name. Therefore, I'm not able to get get those archived messages and copy to their original folder (Inbox). What happens? I had attached a print screen showing that instruction.

Do you see an Archive folder when you login to your account via webmail?

No, I don't. And it's not that it's "hidden", but it's not really available at my gmail account (via webmail).

When logged in via webmail, you'll see an 'All Mail' folder. Are your missing archived messages in 'All Mail'? It's the default archive folder for Gmail.

Hi Christ1, Yeap, I'm missing "archived messages" in "all mail". Could I had deleted or this action wouldn't be allowed? I don't think I had deleted, but I'm trying to identify what is happening. In this situation, one may delete the referred folder, should it still give the alternative to archive messages within Thunderbird? As it would have no folder, shouldn't it not give this option? Tks.

Gmail is different compared to other email providers. If your archived messages cannot be found in any other folder, chances are they are located in "All Mail". Again, "All Mail" is the archive folder, there are no yearly sub-folders. You're not supposed to delete "All Mail", that would erase your entire email archive.

`Undo` should work. On the Edit menu or ctrl+Z