Do they come back when you stop orbiting? SketchUp is designed to adapt its screen to the capabilities of your graphics subsystem and the complexity of your model, and when you zoom, orbit, etc. display features are temporarily turned off, starting with shadows and textures, and on a large model even your objects are shown as bounding boxes only. When you stop orbiting, things should return to normal.
One thing you can try is to make sure that you are running a display driver that supports OpenGL, and then check that Hardware Acceleratio is turned on in the Window menu>Preferences>OpenGL dialog. Changes in there require SketchUp to be restarted to take effect.
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