Original: When the spot was to be photographed again in November 1994 by the Hubble Space Telescope, it had disappeared completely, leaving astronomers to believe that it had either been covered up or vanished.
Simplified: When pictures were going to be taken of the Great Dark Spot again in November 1994 by the Hubble Space Telescope, it had gone away completely, leaving astronomers to believe that it had either been covered up or that it had just disappeared.