Clifford D. Simak 
Choice of Gods [EPUB ebook] 

The Earth’s population was more than eight billion. One day they were there, the next they were gone – all except the guests at a family birthday party, a small tribe of American Indians, and, of course, the robots. Technology disintegrated, the Indians went back to nature, and the rest developed new and extraordinary powers. As for the robots, some went to live with the remnants of humanity, others gathered in their own community and commenced work on the Project, work which was baffling in all its fantastic electronic complexity. Then one day a traveller returned from the stars – and the idyllic existence of the last of Earth’s humans was threatened.