Nobody can tell you anything without proper system info or other technical details, including version of AE, comp settings and so on.
I am guessing that you do not have Active Camera selected or that you have added a camera and moved it out of position so when you change a layer to 3D the layer is out of position. Try opening 4 views or 2 views and look for your 3D layers in those panels.
Ahhh Rick! This had been annoying me for weeks. Everytime I set a layer to 3D in my main comp I would have to set all the layers to 3d in order to see them.
When I noticed this was NOT the case in another comp it started to bother me.