Your chocolate soufflé most likely collapsed because it cooled down. You can avoid this by only taking the soufflé out of the oven when you are ready to serve it. Timing is key! Another reason your soufflé may have deflated is that you did not fold the egg whites and chocolate mixture carefully enough. A gentle fold is the key to keeping all of the air inside of the mixture so that when the soufflé is baked it stands tall. Soufflés trap heat inside the dish which is what causes them to rise. As soon as you take a soufflé out of the oven, the heat begins to escape and this will cause the soufflé to fall naturally.