I also just realized that several extremely critical e-mail folders have vanished. I must have them back immediately.

AND.........now all the e-mails I received today, which should have gone into their individual folders (which have all vanished) have also vanished. What the hell is going on here???

Okay, I did a scan disk and repair, and I managed to get the folders back. But the e-mails that came in today are still missing. And so are all my message filters. Where are those being stored, so I can recover them?

Message Filter rules are stored in files named MsgFilterRules.dat, one per account.

It would be useful to know which version of Thunderbird you're using and on what platform.

Sorry for the delay in responding. I was able to recover everything but that day's e-mails by accessing my daily backup emergency drive, and copying the entire thing (over 1.3 million files) to a new drive, as the original one was overheating and causing all kinds of errors.

Windows 8.1, and the version of Thunderbird is 38.5.0.