A frequent ticket we get that impacts all versions has to do with missing view settings like RealView or the ability to change the display style. The following information explains why those settings may go missing and how to recover them and then prevent it from happening in the future.
When you have an assembly that is in Large Assembly Mode the RealView Graphics, Shadows In Shaded Mode, Ambient Occlusion, and Cartoon will be missing from the drop down menu.
To get these options back you can go to Tools>Large Assembly Mode and deselect it. You can also deselect it from the assembly tab.
To prevent this in the future you can change the number of components to a higher number or disable this. To do this go into Options>System Options>Assemblies