Are you looking for the Mobile Documents folder in the User Library, not the hard drive level Library? Go to Finder and select your user/home folder. With that Finder window as the front window, either select Finder/View/Show View options or go command - J. When the View options opens, check ’Show Library Folder’. That should make your user library folder visible in your user/home folder. Select Library, then Mobile Documents.
Try iCloud.com/Account Settings using a computer browser and go to Advanced at the bottom left. Access Restore Files.