I've put more than 100 hours in creating environments with imported models. All of the sudden all of my imported models disappeared. I also created a model of a table in C4d lite with some slightly beveled cubes as legs. Those also went away. I'm ready to junk After Effects and just use Blender.
What changed on your system? What version of AE? What OS? There is no way we can even guess what is going on without specific details. Software does not stop working unless something changes on your system or you changed the workflow.
This was operator error. The "Axis Bands" filter got unchecked somehow (Not intentionally -- I don't even know what it does). I turned on all of the filters and the models came back. I checked and unchecked filters until I found the 1.
Glad you found the problem.