Try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

No change.

It sounds like your mail files are severely corrupted.

You should create a backup of your Thunderbird profile folder asap. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile

Can you try to move a message in the Inbox to other folders/child folders? If that works move all of the messages in the Inbox to other folders/child folders.

Note, corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Have backed up the Profile and saved it. Have moved all inbox mail to new folder in Local Folders. How do I fix the inbox folder? 'Repair Folder' does not make a change. What have I missed? Thank you.

Once you've copied or moved messages to Local Folders, close TB, open the profile folder, and look in the Mail or ImapMail subfolders for the folders corresponding to the accounts. Delete the Inbox mbox file (the large file with no extension) and the Inbox.msf file, then restart TB; a new Inbox file will be automatically created, and if it's an IMAP account, it will synchronize with the Inbox on the IMAP server.

http://kb.mozillazine.org/Compacting_folders#Real_fix

Done, it worked. Thank you both so much. If necessary in future will this work for other folders that may be corrupted? Trying to figure out how to exclude the Profile folder in Malwarebytes. Not sure how to navigate to the folder thru Malwarbytes. Also could be remnants of uninstalled Norton trial which came with new computer. Thanks a million.

Yes, this fix applies to any folder, although Inbox is the one most likely to become corrupted. It might even be a good idea to perform this kind of 'refresh' on a regular basis to maintain good performance.

You can exclude a folder from Malwarebytes under Settings/Malware Exclusions/Add Folder. The profile folder is usually located at C:\Users\username\AppData\Roaming\Thunderbird\Profiles\xxxxxxx.default

Looks like it is a hidden folder as AppData does not show up after username. I will work on that another time. Thank you very much, sfhowes. Email working well now.