While trying to move my sync'd photos in 'MobileDownloads.lrdata', I moved the catalogue and now even after replacing it to the original file, Lightroom Classic can't find the originals, and the size of the catalogue is now 40kB (previously 86GB). Can I redownload the photos to access them on Lightroom Classic?
I'm sorry, but you have to try to better explain what you did and what happened. In the title of your message you mention moving 'MobileDownloads.lrdata', but in the message itself you suddenly talk about 'catalog'. Those are not the same thing, so what exactly did you move? You also say that what you moved was previously 86 GB, and now it is only 40 KB. That can't be the catalog file, so I assume that is (a new copy of) 'MobileDownloads.lrdata'. So what happened to the 86 GB version? Where is it now? Most likely you only have to replace that new 40 KB file with the original 86 GB file, but we need to know the details before we can advise to do that.
I moved MobileDownloads.lrdata to a folder on my external drive, not realizing that lightroom was open and syncing still. When I saw the error message saying it couldn't find my photos, I quickly tried to move it back, and confirmed a file with the name MobileDownloads.lrdata was in the original folder.
Thinking it was back in the original location I deleted the "moved" file on the extrenal drive, closed lightroom and re-opened it. The error message was still present on the sync'd images, and I went to the MobileDownloads.lrdata in the original location and looked at the info for the file, which shows it to now be a very small size.
I am afraid that I have caused a problem with the syncing, and am wondering how to fix it so that I can once again see my Cloud photos, uploaded from Lightroom on my iPad, on my Lightroom Classic.