How do I fix the Problem With Half my Object disappearing? I am having a slight problem with a moon rover I am creating for a game. When I try to render it (in Blender Game) half the object disappears, or becomes hollow.
Here is a picture from camera perspective of the Object in editor:

 

Here is a picture when I press P to start the Engine:



I am currently using Blender Version 2.74
How can I fix this Error?
Most likely the face normals are facing the wrong way. Select the object that produces error, go into edit mode, select all with A and recalculate normals with CTRL+N.