Suppose that a decrease in the demand for goods and services pushes the economy into recession. What happens to the price level? If the government does nothing, what ensures that the economy still eventually gets back to the natural rate of output?

A decrease in aggregate demand causes the price level to fall. If the government takes no action to counter this, then the actual price level will be below the price level that people expected. Individuals will eventually correct their expectations about the price level. As they do so, prices and wages will adjust accordingly, shifting the aggregate supply curve to the right. For example if wages are sticky, in light of the lower price level, firms and workers will eventually make bargains for lower nominal wages. The reduction in wages lowers costs of production, so firms are willing to produce more at any given price level. Consequently, the short-run aggregate supply curve shifts right. The rightward shift in aggregate supply eventually causes output to rise back to the natural rate.