Game objects are not visible in scene

Hi i am new in unity 3d .I want to add a simple sphere in a Floor.I add Game object Cube and rename to Floor.Then add a sphere in Hierarchy.But it is not visible.i added cube,plane.Noting is visible.What is going wrong?

The are a couple of possibilities. you may have created the sphere inside the cube. This is what happens by default if you don’t move the cube or the sphere. If this is the problem, simply move the sphere until you can see it. The next possibility is that you are not creating things in the view of the camera. If the camera is in the default setup, then first move your object to the origin, then move it to place it in the scene.

If you cannot see an object in Scene view, double click on it in the Hierarchy view, and the scene view will zoom to that object. Alternately, you can select the object in the Hierarchy, hover over the scene, and hit the ‘F’ key.

I discovered an issue similar and resolved by selecting Layers > Default.