I know this is an old thread/question/problem but I am running into this same problem as the original poster and no one ever responded to him. I'm running Civil 3D 2010. I've attached an image of the box shapes that were originally contour lines. This has now occurred in two of my projects that were originally started in LDT 2006.
Thanks for your time.
There is no equivalent contour object in Cviil 3D, and they appear to not be "Object Enabled".
You can go back to LDT 2006 and turn on PROXYGRAPHICS to 1, or explode them to polylines in 2006 so that they can go forward.