Possibly you moved the camera underground?

  If you have a ground plane, that would explain why the sky went black (it's blocking the sky from view and not lit by the default light from above).

  This also ?can?will? cause distortion of the horizon line.  See attached screenshots of a cube and a ground plane.  The top screenshot shows the normal view from above, the bottom screenshot shows the view with the camera moved below the horizon (see dark black sky in upper left thumbnail render, also curved horizon line.)