Empty looking folders after an unclean shutdown are usually an index problem, not data loss. Each Thunderbird folder is two files: a big extensionless mbox file holding the actual mail and a small .msf index telling Thunderbird what is inside. A corrupted msf shows an empty folder while every message sits safely in the mbox next to it.

Check the data first so you can breathe: Help, More Troubleshooting Information, Profile Folder, Open Folder, then browse into Mail, Local Folders. Find the two folders by name and look at the extensionless files' sizes. Megabytes or gigabytes means your mail is right there.

Now rebuild the index: back in Thunderbird right click each empty folder, Properties, Repair Folder. Thunderbird rereads the mbox and rebuilds the msf, and the messages reappear, typically within a minute per folder.

If a folder's mbox file itself shows near zero bytes, that is real damage and the suspects are the overnight Windows update timing plus antivirus quarantining the file mid scan. Check the antivirus quarantine before anything else, restoring from there or from File History gets the mbox back, then run the same Repair Folder.