Sometimes when loading scenes, geometry pieces are visible int he view port, but not in the object list. The levels CAN be changed globally, Pg up/dwn when no object is selected, but Geo CANNOT be altered int he 3d view i.e. the brush tool turns into a dot while over the Geo as if there is none in the scene.
In the attached pic, it lists on the 'grounddetail' Geo which is the ground... another 5 pieces of Geo are visible, but un-editable and unselectable in the scene.
I think the scene has become corrupted somehow...
Many times previously in this project save files have just become unusable when opening, and instantly crash. My confidence in Mud as a production tool is around 5% and personally I am one straw away from switching to Z. Id rather not, but hey... I'm spending more time avoiding bugs than sculpting.