I have an assembly which for some reason is no longer rendering, at all. When I open the part the general shape of the assembly shows black then once it loads it disappears completely. The models are all still there as I can hover over them and their faces and edges show up for selection (see image below). If I open any sub-assembly or part they view fine. I've tried copying and renaming the file, changing the view settings and rebuilding all to no avail.
Also, I like having the appearance panel open. Click on the > at the top of the feature tree. The first column will show if anything was manually hidden. Just click on the icon to bring it back. Screen shot is from a part, but assemblies work the same way.