Did AVG quarantine your inbox?

No, there are no files in AVG quarantine.

Try these in order. Helped me with separate issues:

Issues with mail/mail folders disappearing. The following works for me under Windows 7, 8, & 10 (NOTE: I was looking for specific emails from someone. Not sure what other corrupted files existed.)

• Help->More Troubleshooting Information->Profile Folder (Open Folder)

Should send you to something like C:\Users\{USER}\AppData\Roaming\Thunderbird\Profiles\@@@@####.default-release

• Click on IMapMail
• Search *.msf and delete them all
• Start Thunderbird and let is run for a while (may take a long time if you have many emails) ... I went to lunch. Missing mail/mail folders MAY re-populated under another folder! This happened to the emails I was looking for. I moved the emails to the correct sub-folder.

The auto updater has screwed up some of my setting and glitched Thunderbird Performing a "Clean" reinstall over your existing installation. Please follow these steps:

1. Download the latest version of Thunderbird from http://www.mozilla.org/en-US/thunderbird/ and save the setup file to your computer.
2. After the download is complete, close all Thunderbird windows (Click Exit/Quit from the menu button on the right).
3. Run the setup file downloaded in item 1

I hope this helps.