The 3d model that I am using with the car wizard has suddenly stopped working on me. It was deleted from both of my cars and the ‘Import Settings’ and ‘Reimport’ options for this object are now greyed out. I cannot add an instance of this object to my scence either. I have submitted a bug report, but I am wondering if part of it might be ignorance on my part rather than a bug. It seems to me that I have seen these options greyed out before. Is there a reason that Unity should do this?
Did you perhaps create a prefab and then delete the original model? Even if you use the mesh in a prefab, you need to keep the original asset (the one with the cube and sheet of paper icon) in order to keep the mesh data. The prefab only stores a link to the mesh.