Welcome Rick Hughes,
I understand the concern, especially with more than 10,000 pictures involved. Before trying to move anything back, let's first determine where Windows has actually stored the files, since “Gallery” can display pictures from different locations without necessarily being the physical folder where they are stored. Microsoft describes the Photos app as bringing photos from different locations together in one view.
Please try these steps:
- Open the “Photos” app and find one photo that was previously inside one of the now-empty folders.
- Right-click the photo and select “Open file location” if that option is available.
- If you do not see that option, right-click the photo and select “Properties”, then check the “Location” shown there.
- Copy that location and open it in “File Explorer”. Check whether your other missing pictures are stored in the same place.
For now, please do not move, delete, or re-import the photos, especially since there are more than 10,000 files.
Once you find one of the pictures, could you tell me the “Location” path it shows, for example C:\Users\...\Pictures\... or a OneDrive location? That will help determine the safest way to restore the original folder arrangement.
Microsoft also documents how the Photos app imports pictures and allows an import location to be selected: Import photos and videos from phone to PC
Looking forward to your update.