The sky turned green the day the aliens invaded. Drops of tiny robots made of nanotechnology fell from the sky end everyone they touched was incased in a hard shell frozen in temperature and frozen in time. Anyone who touched the frozen were themselves infected and frozen as well. This was the nano-plague.  The alien spaceship landed on the moon and teleported aliens onto the surface in mechanized suits to spray nano-plague on anyone who had been missed by the rain. The suits were strong and impervious to rifles and it would take a round from the main gun of a tank to take out an alien.  Unfortunately for the aliens, Earth had plenty of tanks and the aliens were swiftly defeated. Theirs had been a farming expedition designed to subdue primitive sentient beings for harvest and the alien minds ill-equipped for war though their technology was far superior. Unfortunately for the humans, more than half the population of Earth (and many of the smarter animals) had been frozen. Five years after the GD (in the year 5 GD on the new calendar) one scientist succeeded in unfreezing his first plague victim.