If one were to accept the mini-comics, then there was a great age. Magic and technology mixed in ways still unimaginable to create a society that had no equal.
Then something happened. War. No one knows what started it or what the sides were, or even why they fought, but they did. It was terrible and it destroyed them. It was so terrible it broke time and space.
Those who lived on did so in the ashes of the aftermath. Only small pockets remained, bastions of technology that had escaped the fires of war. Magic suffered a similar fate. The world descended into barbarism and chaos. Knowledge and peace was replaced by savagry and survival of the fittest.
The Goddess/Sorceress had been charged with watching over two resources, one of which was in a cave of artifacts (He-man would get the Harnesses and the Battle Ram from it), the other was Castle Grayskull. Skeletor arrived to Eternia via a tear in time and space, forcibly dumped on Eternia with no way home.