I solved the problem. I dragged the file from my backup to my desktop 1. Opened Thunderbird, 2. Click on the menu button or menu bar. Very top of computer screen 3. From the Help menu (last selection Right), click Troubleshooting Information. A window will open. 4. In the Application Basics window, click the Show in Finder box, about half way down on the right side - grey colored area. 5. Found the last entry that matched the ID label of the file on desktop. Clicked on it and clicked on update when prompted ( I think that is what they called it). I clicked Yes. Then I closed Thunderbird. 6. Reopened Thunderbird, signed in to thunderbird and to me miraculously the files were there! I have no real knowledge of why or how but this seems to be what worked for me. Best of luck to you.