Answer
If nothing happens, then the aggregate demand curve will shift to the left. This would decrease output and increase prices.
Work Step by Step
If the Fed decides to stabilize aggregate demand, then the Fed needs to increase the money supply. This would likely shift the aggregate demand curve back to its original curve. (The increased money supply would cut the interest rate, increasing investment.)
If the Fed does nothing, Congress would want to stabilize aggregate demand by increasing the money supply. However, the Fed directly controls the money supply, so Congress would need to increase the quantity of money another way (such as tax cuts).
