"All my props have disappeared!"
One day you open your project and it appears all your props have disappeared Your Library looks something like this: The first thing to do is to go to the Help menu and scroll to Check For Program Updates. If any files are indeed missing, this will replace them.
If there is no change, go to the TOOLS menu, choose MANAGE SUPPORT FOLDERS and go to OBJECT LIBRARY and see if, in fact, that is where the objects are located.
Still nothing? Then it is likely the props list area has been resized so the categories aren't visible. Here's a video showing how to fix that: