Check your oven temperature, by using an oven thermometer to make sure that your oven is at the correct temperature.
The wrong temperature can cause the cake to rise too far and collapse on itself.
Ensure that the cake is cooked through by using a skewer or thin knife inserted in the center and seeing that it comes out clean. If the cake is undercooked in the middle, it will cave in.
Never open the oven door in the first half hour of baking as the cold air will interfere with the rising of your cake.