Please share a screenshot of your full ae interface so we can see exactly what is you setup.
I bet you set this up using one of AE's alternate views and not the Active Camera. Go back to the problem comp, check your views, and if it's NOT set to Active Camera, that's your problem.
Most likely reason for the layer disappearing when you change the layer to 3D is that you are not looking at the Active Camera view. Active camera is the only view that you can render. All other views are just for setting up 3D layers in AE's 3D space. The other likely cause of a layer disappearing is that you are in the Active Camera view but you have added a camera to the scene and have it pointed away from the layer you are having problems seeing.
To learn AE please start here:Basic AE