Occasionally, when using the push/pull tool, SketchUp will delete your front face and create a hollow shape (see image to the left.) To avoid this, simply hold down the control key to force SketchUp to keep the front face of your shape.
This may leave you with a front face that is a different color than the rest of your model. This is because this face is showing its back side instead of its front side.
If this is the case, simply right click on your face and select “reverse faces” to get the white side of your face to show up.