Before doing anything else, I would free up quite a bit of memory. 1. I would restart thunderbird, and see what things look like then. 2. If that doesn't work, I have sometimes thought a folder that should have email in it was empty, but it turned out that it was set to only display unread emails or there was some unfound term in the search area, so check for that. This probably won't solve the missing folders problem, though. Could it have moved those folders into other folders as subfolders? 3, If that doesn’t work, I would save (not move) a copy of the thunderbird data, which in my system is at C;, Documents and Settings, NewUser (this name will vary on your system), Application Data, thunderbird off somewhere so if what I am about to suggest makes things worse you can at least get back to the current state. In newer versions of Windows than I have it may be somewhere else. Then I would restart thunderbird and tell it to compact folders. Hopefully that will recover stuff. 4. I am out of ideas, but in the future, you might periodically save the thunderbird data as in step 2, so that you have a backup in case things go west again.