Let's get the easy stuff out of the way. I'm on an MBPr (late 2013) Running macOS Sierra and LR 2015.12 release, camera raw 9.12 with all the latest updates.
Yesterday I logged into lightroom.adobe.com and noticed that I have a web gallery set up from a while ago. (I was a Lightroom CC member a few years ago but canceled the account a few years ago and restarted it a month ago.) Not knowing what was up there I did an export and copied everything to my drive. Now the Web said I had ~70,000 photos while the catalog on my machine only listed a little over 20,000.
Now at this point, I'm not sure what to say as things are so screwed up. The catalog shows the files as supposed to me on the main hard drive but is missing. The files and the old catalog was pointing to the files on a removable drive (which the files are still there. The catalog shows all photo's as 70110 and when I look through the list I start to see 4 copies of some files, 2 of others and no copies. The file structure and photos that I had are still on the drive where I had them but the catalog shows them on the hard drive, where they are not. I also have a folder from the export which seems to have 170,119 items which seem to be primarily 9 KB HTML files.
Checking the catalog settings I do not see where the catalog is located but not seeing any backups after 2014, which is probably when I canceled LR the last time.
Sorry about the long post but there seem to be so many things that have been screwed up I don't know what to say or start with.
Let me just correct something, I actually went to the web tab in the Lightroom app and not the web site.