Most likely your mail files were corrupted. 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 Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird You can restore messages from a recent backup of your Thunderbird profile folder, assuming you did create a backup in the first place. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile