Changed Thunderbird from POP to IMAP

Do not delete your POP account until you are sure. To get your lost emails, you need

1. restore your POP profile if you still have it https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

or 2. You have set "Leave messages on server" for your POP account, then you should still have the emails on your server.

I created a user profile for IMAP and all was OK....Until I deleted the profile for POP and I lost all e-mail folders that I didn't move to IMAP.

Did you really create two Thunderbird profiles, one profile with the POP account and then a second profile with the IMAP account? Or did you have one profile with the POP account and then set up an IMAP account for the same email address, in order to copy messages from the POP to the IMAP account? Normally, when you delete a POP account in Account Settings, Thunderbird prompts to remove only account information but not the message data. We don't know whether you checked "Remove message data" and probably you don't remember.

To verify if your message data are still stored in your Thunderbird profile go to >> Help > Troubleshooting Information > Profile Folder > Open Folder: in the profile folder go to Mail and verify its contents. Does it contain an account folder something like "pop.xxx.xx" and does this folder contain mailbox files such as Inbox, Sent, Drafts, Archives, Trash?

Please forgive me as I have confused profiles with account. When I deleted the POP account I chose to remove ONLY account information. And I am pretty sure I did not check "Remove message data"! I have tried the "Help > Troubleshooting Information" "Profile Directory" "Open directory (local drive)" I then went to Mail and indeed this folder contains "pop.mail.com" and in there seems to be everything I have lost. This seems to be VERY GOOD NEWS!! Now, how do I get this stuff back?

Good. Just backup it first.

In the help https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

I think you can try to copy the folder to your profile (you need create a new POP profile) Restoring a profile backup

You can restore your data using Thunderbird's import tool, or the steps below.

   Shut down Thunderbird.
   If your existing profile folder and profile backup folder have the same name, simply replace the existing profile folder with the profile backup, then start Thunderbird.
   Important: The profile folder names must match exactly for this to work, including the random string of 8 characters. If the names do not match or if you are restoring a backup to a different location, follow the steps below. (see above link)