Let's utilize what we learned in Econ.
Now, the demand was say, at 10 million barrels a day, for the US, and the supply was 10 million, and the cost was 1.50 a gallon. Then, 9-11 happens, oil demand decreases (planes on the ground, noone wants to go anywhere, etc, etc), and supply is still at 10 million barrels. Suddenly there's a surplus of oil, and the market clearing price (price where demand meets supply, previously at 1.50) drops lower and lower. Now, when supply is lowered along with demand, or if demand goes back up, the price rises. Simple as that.