SOLVED Thousands of files and folders disappeared, apparently date related

[misterfish]

Win 10 on a Dell desktop, Firefox and Thunderbird up to date.
Two days ago. Downloaded a later version of QBTorrent, installed no problems. Everything working. Powered down overnight.
The morning - disaster -
Firefox tabs reverted to March 2022.
Gmail accounts (all five) blocked to Thunderbird,
90% of Thunderbird files and folders disappeared.
About 99% of QBtorrent files disappeared, QBtorrent itself reverted to earlier version..
Downloads after 15th March disappeared.
Just about everything dated after 15th March 2022 has gone, including all "recent" lists.
Not one of the missing files is in Recycle Bin.

Good news - email accounts with SFR.fr and Luxembourg online unaffected, I had backups of Office, QBtorrent, and emails are still present on gmail's site.
Bad news - gmail emails (around a thousand) will have to be individually forwarded to myself to recover them. T'bird folders will have to be replaced manually.

Question: what could have caused such a catastrophe? What triggered the loss of so many files?
Question: any idea of where the Thunderbird folders can be lurking?

 

[misterfish]

Solved the problem. I noticed that the computer was booting very, very slowly, and the spinning HDD was churning away, which reminded me of why I changed to an SSD. Why was the HDD churning? Aha, somehow, when booting up the OS automatically chosen was from the previous installation on a spinning HDD, rather than the new installation on the SDD. When did I change over? 15th March!
So changing the boot order took me into the later version of WIN 10 and all my files and recent lists were recovered.
It was a bit like Schroedinger's computer, two parallel worlds with different histories!