next time I opened Sent it was completely empty.

Did you see any message about compacting?

Try to inspect the 'Sent' file in your profile. What size does it have? It's a text file, so you can open it with a text editor when Thunderbird is closed.

To get to your profile folder: At the top right of the Thunderbird window, click the menu button , then select Help > Troubleshooting Information > Profile Folder > Open Folder.

This will open the profile folder in Windows Explorer.

I also have Outlook installed so I opened that and the Sent folder is empty there too.

If your accounts are set up as IMAP on both, Thunderbird and Outlook, this is exactly what happens. Changes are synced via the server. This is why backups of the profile folder are important. In worst case you can restore the missing messages from a recent backup, assuming you took the time to create one. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Thank you, Christ1. I got to the Profile Folder but couldn.t see anything about size of Sent File (the only folders were Crashes, ImapMail, Mail amd minidumps and they didn't have that info). Probably you're right about the problem resulting from running both Outlook and Thunderbird - and of course I hadn't backed up.

Look underneath ImapMail/<your account name>.

Found it. Many thanks. Have now been able to read the contents. Your help greatly appreciated, And now I'm backing up.

If the damaged "Sent" file can still be opened and read as a text file, can the damage be repaired with a text editor, so that the contents are once again accessible within Thunderbird?

Alternatively, can selected messages be salvaged and transplanted to a new "Sent" folder? This problem is not rare - it often happens unnoticed for a few days or weeks until I go looking for a message, and suddenly discover that everything in a folder or file is gone. I do keep backups, but after the passage of time, it may be hard to determine just when the mishap happened, and then locate a backup from just the right time.

(Note that in the attached screenshot the message headers remain but the contents have all disappeared. But sometimes both the contents and the header will disappear.)