Thunderbird has a built in repair for exactly this, and understanding what it does explains why it usually works, since the problem is typically the folder's index rather than the messages, so the repair then the deeper options:

The Repair Folder tool, the first and usual fix: Thunderbird stores each folder as an mbox file of messages plus an msf index file, and when the index and the actual messages fall out of sync, the folder misbehaves, messages missing from view though present in the mbox, wrong counts, display problems. Right click the folder, Properties and click Repair Folder, which rebuilds the index from the mbox and the messages reappear correctly since they were there all along, just misindexed. This resolves the majority of Thunderbird folder problems.

What Repair Folder actually does, so you trust it: it deletes and rebuilds the msf index file from the mbox message store, a safe operation because the messages live in the mbox and only the index is regenerated, so repairing a folder does not risk the messages, it re reads them. This is why it is the safe first thing to try, nothing to lose.

The compact step, for folders bloated or with deleted message gaps: Thunderbird marks deleted messages rather than removing them immediately and a folder needing compaction can misbehave, so right click the folder, Compact or File, Compact Folders, which actually removes deleted messages and tidies the mbox, sometimes clearing problems that repair alone does not.

The deeper corruption case, when repair and compact do not resolve it: a genuinely corrupt mbox file, rather than just a bad index, needs more and the routes are restoring the folder from your profile backup if you have one or extracting the readable messages with the ImportExportTools NG add-on which can often read a damaged mbox and export what survives. The order is Repair Folder first as the safe usual fix, Compact next for deleted message and bloat issues, then backup restoration or add-on extraction for true mbox corruption, with backing up the profile folder before deeper attempts since those touch the actual message store rather than just the index.