I am on 10.12.6.
I know I should have had a backup.
I have not used OneDrive locally for years, not launched the app etc, but I have a local OneDrive folder on my Mac where I stored things in the past and at one point synced them. I can't remember what stopped working with it, but I eventually just uploaded to the web when I needed to. But I also kept adding files to that local non-synchronizing folder. A mess, I know. I had OneDrive files on the web that weren't on the Mac, and vice versa. I just started treating the local folder like my local home folder.
Anyhow, I have studiously avoided doing any OS or app updates. I finally got tricked. The notification thing that pops up for the mac app store is usually one I can avoid, but I accidentally clicked it, and it did some OS updates (haven't restarted yet) as well as updating OneDrive which I couldn't find a way to cancel.
I have no idea how, since OneDrive isn't running, but several of the folders in the local, non-synchronized OneDrive folder that were years old now show a modified date of Today and a lot of their files are missing. Those are ones that I do happen to have synchronized a long time ago, and I can see that there are files on onedrive.com that aren't on the Mac, somehow they were deleted locally and their folders date was changed to today.
I can tell there are other missing folders that I had added to the local OneDrive that had never been synchronized.
I tried explaining this to Microsoft tech support, and after trying to explain it over and over they finally asked if I had checked my iCloud . . .
Basically, I think something in the installation process touched the folder--maybe it tried to start syncing again? There is nothing in my local trash or the recycling bin on onedrive.com
Any ideas on where the files could be?