Check you have this setting:

• 'View' > 'Folders' > 'All'

Then do this:

• Menu icon > Help > More Troubleshooting Information
• Under 'Application Basics' - Profile Folders - click on 'Open Folder'

window opens showing contents of your profile name folder

• Exit Thunderbird now - this is important

Scroll down and delete the following files:

• folderCache.json
• folderTree.json
• virtualFolders.dat
• session.json
• xulstore.json

Click on 'Mail' folder

• delete 'smart mailboxes' folder

Start Thunderbird.

If this fixes your problem please mark as Chosen Solution. Report back on results.

Thanks for the suggestion, but I was able to resolve the problem another way. I looked at the details of each email account (Menu > Tools > Account Settings), and found that the missing email account was listed in the navigation bar on the left hand side of the Account Settings page. I clicked on the missing email account to get details and discovered that for some reason the "Outgoing Server (SMTP):" entry had a generic SMTP address instead of the correct SMTP address for that email account. Why this would have changed, I cannot explain. I put in the correct SMTP address and then closed TBird. When I opened TBird again, the missing email account now showed up correctly on the main menu. Perhaps it was merely closing TBird and restarting it again that permitted the missing email account to show up again, I can't say for sure.

 So the problem seems to be solved. Thanks for your quick response and help.