This can happen for a few reasons. 1) If you underbake the cake, then the middle will sink. Make sure to use a toothpick to check the doneness of the cake. 2) If either the baking soda or baking powder is expired, then the cake can sink because it isn't properly leavened. 3) You over mixed the batter. Be sure to follow each step carefully so as to not allow this to happen.