#2 was my first thought too. Although if you get that wrong, it typically leads to a dragging brake instead of no braking.
Another possibility: When you put the brake calliper back in place, you typically have to widen the gap between the brake pads a bit in order for things to fit. It may require multiple applications of the rear brake for that gap to narrow again.
Like Gert said, picture please. Also, tell us what you feel when applying the rear brake. Is it completely solid, or offering no resistance at all? Does it feel spongy?