Is this an IMAP account or POP3? Is your "Miscellaneous" folder a Local folder or an IMAP folder? They method to (possibly) recover the folder depends on the answers to these questions.

IMAP account. I have emailed Virgin Media and asked if they can restore the folder. Waiting for reply. Is there another way if they dont help ?

One thought occurs to me - is it possible that you accidentally drag-and-dropped the miscellaneous folder into another folder? If so, you can drag it back into proper place. If not...

When you log onto Virgin's web mail, is the folder visible there? If so, confirm that Thunderbird is subscribed to it: right-click on the account name in Thunderbird's left-hand accounts/folders list, and choose "Subscribe". Click the "Refresh" button, then confirm that the folder is listed and has its box checked.

If that doesn't do it, check to see if the folder file still exists on the local hard disc.

This article will help you find your Profile. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

Then navigate to the ImapMail folder, then identify the folder representing your Virgin account and enter it. Here you will find individual files for each folder. There will usually be a pair with the same name, one with the file extension MSF (this is the message index and can be ignored.) Hopefully you will find one called "miscellaneous". That could be imported back into Thunderbird. So, if you find this file, let us know and we can go on to the next step.

By the way, it's good practice to backup your Thunderbird Profile regularly if your email is important and irreplaceable.