I wrote this partly for Sandwichman, and mostly I wrote it because this same question crops up time and time again. It's a very old question, but it always looks like a new question if the technology is new enough. People in caves were probably arguing about whether 3-D printing robots flints would cause mass unemployment. And my answer is an old one too. There's nothing new in this post.
Take a very simple example. Labour is the only input, there's a constant returns technology, and labour produces one apple per hour.
Start at full employment, where everyone works 40 hours per week, and nobody wants to work any more than 40 hours.
Suppose technology changes, so that productivity doubles. Labour now produces 2 apples per hour. What happens? Will employment stay the same, so output doubles? Will output stay the same, so employment halves? Or something in between?
Is mass unemployment a possibility?