There are several thousand over almost 6 years.

Are you saying you did accumulate several thousand messages in Inbox on an IMAP server over 6 years?

If so, thank god it worked that long.

Do you have a backup of your lost messages?

wish I did. I tend to keep everything in my inbox rather than moving emails out into their own folders. My Inbox has been deleted, the rest of my folders, have not. The IMAP server belongs to my ISP and has ooddles of space left. As far as I was aware, the emails stayed on the servers and TBird was just pulling down a copy. It wasn't deleting the email from the server the way it does for my POP3 accounts. So I was extremely surprised to see my inbox get deleted on both my PC and the webmail account, especially as I was not trying to delete the Inbox, just the emails marked for deletion.

This is most likely what happened. You deleted a number of messages in Inbox which then triggered a compact folder action. See http://thunderbirdtweaks.blogspot.com/2011/07/compacting-what-is-it-and-why-must-i-do.html http://thunderbirdtweaks.blogspot.de/2011/07/compacting-what-is-it-and-why-must-i-do.html

Accumulating messages in Inbox makes it more susceptible to corruption. The most likely cause for corruption is anti-virus software messing with Thunderbird mail files.

Therefore create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Whether you can restore any messages depends on what's left in your Inbox file. What's the size of the file on disk?

where would the file be? Not too certain where TBird puts everything. Would this explain why the server lost the emails as well? thanks for all your assistance, I appreciate it.

read the links so have found the file locations. my current inbox is a few KB only. On old inbox file from May is 1.6GB and its SBD folder is 3.5GB

my current inbox is a few KB only.

Would this explain why the server lost the emails as well?

Yes. Your messages got deleted locally, presumably due to mail file corruption. And this change gets synced with the IMAP server. With IMAP it is therefore important to also have a backup of all your messages, even though they are stored on the server.

On old inbox file from May is 1.6GB and its SBD folder is 3.5GB

That looks promising. What does 'old inbox' file mean? Is this a backup? Please explain what exactly you have and where is is located. A screenshot of Windows Explorer with the files/folders, and the full path visible would certainly help. http://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

The old inbox is not a backup. I think it happened because back in May, my ISP moved from Gmail servers to their own or another server supplier. Attached are screen shots of the old and new file locations. Could these be imported in some way?

Before doing anything else, create a backup of your Thunderbird profile. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile

The account we're talking about is the virginmedia.com account? Please confirm.

It is the virginmedia account. The phonecoop accounts are POP3 and are not a problem. Backup has been made.

At the top right of the Thunderbird window, click the menu button > Options > Account Settings.

Select the virginmedia.com account in the left pane - Server Settings.

At the bottom is a field 'Local Directory'. What path does it show?

C:\Users\User\AppData\Roaming\Thunderbird\Profiles\e7a6d27s.default\ImapMail\imap.virginmedia-1.com

To recover you missing messages from the folder C:\Users\User\AppData\Roaming\Thunderbird\Profiles\e7a6d27s.default\ImapMail\imap.virginmedia.com I'd sugegst the following steps.

Close Thunderbird.

In Windows Explorer navigate to the folder above (imap.virginmedia.com).

Copy the mail files (files without a file extension) to a temporary location, e.g. your desktop.

Rename them so that you will not overwrite any existing files in the final destination folder. Example: rename INBOX to e.g. INBOX_recovered The same goes for 'Sent' and 'match'.

Move the renamed files to C:\Users\User\AppData\Roaming\Thunderbird\Profiles\e7a6d27s.default\Mail\Local Folders

Start Thunderbird. You should now see a folder 'INBOX_recovered' underneath 'Local Folders'.

You may want to turn off your anti-virus software unless you did follow the instructions at https://support.mozilla.org/en-US/questions/1138350#answer-915333

The next step will be to upload the recovered messages to the IMAP server if this is still needed.

If something very bad happens you can recover your profile from the backup you created. Always make sure your backup is up to date.