I'm sorry I have no advice on how to recover the emails that have disappeared. I hope someone else in this forum may help you. I'm writing to explain that trying to keep 19,000 emails in your Inbox is a very risky approach. The way Thunderbird works, all the messages in a folder are stored on your computer as a single file. Every time a new message is added or one is deleted (which happens a lot in the Inbox) the whole file has to be rebuilt. The more messages, the longer the file, and the greater the likelihood that at some point the file will get corrupted and errors creep in. I'd encourage you to read this: http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29 Lots of good advice there, including in particular the principle of keeping your Inbox as small as possible. If you have old emails that you want to keep, move them into other folders where there won't be many changes and the risk of corruption is minimised. And back up your Thunderbird profile regularly, keeping a copy separate from your computer -- then you won't face this nightmare again.

It sounds like your mail files are corrupted.

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 or which are gone already. In that case you'd need to restore them from a recent backup.

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