Catalog has the files, but aren't synced.

Are you asking about all those parent folders that show '0' images in the folder panel?

When you click the disclosure triangles do you see photos in sub-folders?

What setting do you have in the Library Menu for-

That ? mark next to Multi. Missing folder. see:

• https://www.computer-darkroom.com/lr2_find_folder/find-folder.htm

Many thanks for the kind and quick response, but I've realised I hadn't framed the question correctly.  I'm not trying to sync images outside of Lightroom classic, I'm trying to get the images inside my pc (shown under All Photographs) to appear in Images (G:) in Folders.  They were all there 2 days ago, but have gone.  The files of images themselves are intact, arranged as shown in the screen shot below.

I back up daily, but the latest .data file, the 2nd one Adobe added, seems corrupt.  Should I try to find a back up of that and re-install?  And if so can you point me in the direction of how, please, as I'm lost and worried!

Lightroom-Classic will not automatically sync 'All Photographs' in a LrC catalog.

You can only sync photos that you place in Standard Collections, and mark those Collections to Sync.

If you expect all 37094 photos to sync to the Cloud then they need to be in Collections that syncs.

SYNC SETUP IN CLASSIC

And be aware that LrC syncs photos to the Lightroom Cloud as smaller proxy files (aka Smart Previews) that are limited in pixel dimensions to 2560px long edge. These proxy files in the CLoud to not occupy any of your Cloud space (that might be 20GB.)