Perhaps you could offer some bread crumbs of information for someone to work with you.

Was the folder part of an imap account or a sub folder of a pop account or in "Local Folders"? Was the mail provider perhaps outlook/hotmail/MSN or even Google or Yahoo? This is a leading question where I expect an answer in more than "No". Who is the mail provider? What was the actually name you used for the folder? There are all sorts of punctuation type things in folder names that can cause problem. If the language used is not English were accented characters involved?

The Folder was called Orders 2021

   Orders 2021  <----- this folder is the one that went missing
   Orders 2021 Comms

and it had different email address in it as I put MY Orders in it, it is a Subfolder. English I have had this for 2021 and I do these folders for each year since 2010.

If it's an IMAP account, right-click the account in the Folder Pane, Subscribe..., Refresh, and see if the missing folder can be subscribed.

Hello They are POP accounts and does not belong to ONE account it's where I put all Orders from each email account I have in a Folder and then a Subfolder, All was good until my last update and then it was gone and NO *.msf file in my Thunderbird directories. Thanks Anyway.

Was the missing folder a subfolder of Local Folders? Help/Troubleshooting, Profile Folder, Open Folder, close TB. Is the mbox file (large file with no extension, named Orders 2021) in Mail/Local folders or Mail/<popservername> or an sbd subdirectory?

I have in the Mail/Local Folders Orders 2021e15b528d.msf 4KB 16/08/2021 Orders 2021e15b528d 640 KB 06/01/2021

Mail/Local Folder/Orders 2021e15b528d.sbd 4.18 MB ‎ 15/08/2021 Created 06/01/2021 This Directory has *.msf and non extension files of the same names.

What happens if you close TB, delete all the .msf files, copy all the mbox files from sbd folders to Mail/Local Folders, and rename Orders 2021e15b528d (640 KB) to Orders 2021, then restart TB?

I end up with a empty Orders 2021 Folder.

If I open Orders 2021e15b528d (No extension) with Notepad the emails are in there, so I need to convert to msf file or something like that ( I Hope )

An msf file is just an index of the mbox file that contains the message content. If the mbox is not irreparably corrupted, TB will automatically rebuild the msf file when it restarts.

If the folder is empty in TB but the messages appear in Notepad, change the X-Mozilla-Status header to 0000, save it, then restart TB:

https://support.mozilla.org/en-US/questions/1155024

So much wanted that would work but No.

From - Fri Jan 01 10:59:55 2021 X-Mozilla-Status: 0000 <---- (that was 0009) X-Mozilla-Status2: 00000000 X-Mozilla-Keys: Return-Path: <202101010343065ed75a27a8594a47b2edba153f40p0eu@bounces.amazon.co.uk>

You might have some progress using an mbox repair app. I haven't used any of them:

https://www.mailsware.com/mbox-repair.html

https://www.bitrecover.com/mbox/fixer.html

If an mbox is converted to eml files, those files can be imported to TB by drag and drop.

Thanks for you help but I'm just going to accept they are gone. Thanks Anyway