Lethe

"'Lethe was one of humanity's first successful communities in another solar system. It was thriving, a jewel of our burgeoning galactic civilization, until ten years ago when the entire population, almost 50 million people, vanished.""You have been tasked with investigating the mass disappearance,and your ship is about to make landfall. Begin by establishing Habs in the abandoned cities to get them up and running. From there you can begin to re-populate the planet, and try to figure out what happened.'"