What s wrong with the polygons of my model why are all those polygons transparent in my model?
What has happened is the normals on you faces have flipped.  This will sometimes happen when you scale a mesh negatively or you extrude a face in a negative direction local to the face.  One way to see these faces is by disabling backface culling.  But I would not recommend this, because all it does is makes both sides of the faces visible in the viewport.  When you render you will still get dark looking faces.  For a permanent fix, go into Edit Mode, press A to select all faces.  Then select recalculate normals under Shading/UVs in the tool bar, or just press CTRL+N