there is no problem when the camera is away, but as I zoom in to the game, the images disappear and I can't see the game properly. What is the reason ?
This is most likely caused by the camera's clipping planes.
The Near and Far Clip Plane properties determine where the Camera’s view begins and ends. The planes are laid out perpendicular to the Camera’s direction and are measured from its position. The Near plane is the closest location that will be rendered, and the Far plane is the furthest. -Unity Manual: Camera
If you need to see objects that are closer to the camera make the near value smaller.