If someone sends you an email without filling in the subject line, that space will appear to be blank in your message list. You can still click on the message to see it.
I notice 27 messages in a row, all having the same time stamp. All from the same sender?

Ask the sender to fill in the subject line.
There is an add-on that you can use to manually add a subject to a message that does not come with one. https://addons.thunderbird.net/en-US/thunderbird/addon/headertools-lite/

It sounds like your mail files are corrupted. First of all create a backup of your Thunderbird profile folder. https://support.mozilla.org/kb/profiles-tb#w_backing-up-a-profile

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

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup done prior to the corruption.

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 https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile