The Inbox has become corrupted. '''1.''' [https://mzl.la/2lBfidM#w_backing-up-a-profile Backup the Profile folder]. In fact, you should do this regularly in any case. Backup the entire computer regularly. '''2.''' Right-click the Inbox, choose '''Properties''', then click the '''Repair Folder''' button. '''3.''' Select the Inbox, then Compact Folders. Menu: File: Compact folders. '''4.''' The Thunderbird Inbox is the most used folder, with e-mails going into and out of it all the time. Thus it is the most vulnerable to being corrupted. Therefore you need to treat it carefully. Please read and follow the advice in the article [http://kb.mozillazine.org/Keep_it_working_-_Thunderbird Keep it working - Thunderbird]. If you don't follow this advice, your Inbox will continue to get corrupted occasionally, and you may lose e-mails when it happens.