Thunderbird stores mail in a single large file. It also stores a database that is used to populate the user interface lists, occasionally these two things get out of sync. So right click the folder, select properties and then the repair button. This will initiate a re index of the stored email. It may see many simply disappear from the user interface, but the new index does not affect stored mails, only the indexes used to look them up. Which may currently by pointing to nothing, hence no content is displayed.