The US housing market in late 2008 provided a classic example of a "change in demand" and a "change in the quantity demanded."
- The demand for housing declined, shifting the demand curve to the left.
- The supply curve is definitely not horizontal.
- Thus the drop in demand eventually led to a decline in the price.
- And this lower price led to an increase in the quantity demanded along the new, lower (leftward of the old one) demand curve.
For the BBC's coverage, see this [h/t to Brian Ferguson].