Beyond what Ian said, I have a possible two other solutions to this problem if the data isn't there already as described.
What you need to do is go to your Thunderbird profile folder. It's been described above, but you need to go to the folder its self in this path: C:\Users\*username*\AppData\Roaming\Thunderbird\Profiles\*Thunderbird profile name*
Replace *username* with your computer's username in Windows, and *Thunderbird profile name* will be the wired nonsense written name for the Thunderbird profile name folder its self.
Now once you find the Thunderbird profile folder which may have a name like, "apt3p8oq.default", right click that Thunderbird profile folder and chose properties. Now select the Previous Versions tab and pick a version prior to the masterful Thunderbird screw over. LOL
Hopefully this restores your previous Thunderbird profile with all data intact. The reason why this is possible is because of a Windows facility called shadow Copies. I sure hope you have not turned that service off.
Once you have your precious data back, learn to become a major backup freak like I am. With CD keys, software keys and all that rot, I back it all up in a text file that's placed in My Documents folder and on my second attached hard drive. I also burn this txt file to DVD/RW and Blue-ray ( I have a Blu-ray burner). Then I also place a copy on my laptop sitting here on the desk. Another copy is on a USB drive and then finally I upload it to my box.com cloud drive. I even use Amazon AWS S3 storage for things as well. Except when I upload stuff to the cloud I encrypt it myself with 7Zip's SFX archive. And this is really going out there, but for some data like my website backups I'll not only compress and encrypt the backup with a SFX archive, but I'll then copy that archive into a Truecrypt encrypted container. You may wish to use Veracrypt the now fork of the defunct Trucrypt. All passwords are 30+ characters long and I use Keepass for password management. But Keepass may have a learning curve and you absolutely MUST backup its database all over hell with new entries being added. If you're not computer savvy then something like Lastpass or Dashlane would suffice, but I don't trust somebody else's server no matter how much security they say they have. That's just how I roll. I also use the Keepass2Android App which uses the same database scheme. So I can use 30+ character passwords on my phone. Just something to think about. If you're interested in using Keepass, I'll tell you how your database should be crafted to prevent someone cracking the database. I know how it's done and I done it myself from a test database meant for this purpose from a website.
Now beyond all that with important data, I do periodic full disk clones. Not images, 1:1 disk clones. The clones are written to an external USB connected hard drive. That hard drive is then placed in a sandwich bag and stored in a ~$35 fireproof safe. So are the DVD/RW and Blu-ray disks I talked about. Now these ~$35 fireproof safes can be broken into very easily, but that's not why I own two of them. I bought them for water and fire proofing my data should God forbid disaster strike. Use a sandwich bag or other types of bags for USB sticks, hard drives, optical media and for text documents use plastic sleeves, because if there's a fire the contents inside the safe will get moist. And keep the safe flat, not in an upright position. Just all something to think about.
Now if you don't have any shadow copies or if the shadow copy didn't work, you're now going to have to pray like a massive preying mantis a data recovery software works. There are many out there. If you go to snapfiles.com (my GoTo site), search for the word recovery and you'll see plenty listed. What you'll do is have the software just search in the Thunderbird username folder path and hopefully you can pull up intact data. If you're having to use this method, then minimize what you do on the computer now with programs and data. Especially data writes. There's always going to be data writes in the background, but eliminate it as best as possible otherwise you run the risk of the data not being 100% recoverable.
