You let it rise for too long. When bread rises for too long (over-proves) directly before going into the oven, it will fall because it's risen as high as it can, so it will fall down and sink. You could have also put too much yeast or sugar.

If you live at a high altitude, you will need to adjust your ingredients for the thinner atmosphere.

For example, I live at about 7,600 feet. Close to sea level, you'd want to put 3 teaspoons of yeast and 3 tablespoons of sugar into your dough. But because the atmosphere is thinner the higher the altitude, there isn't as much air pressure, so you need to reduce the yeast. I only put in 1.5 teaspoons of yeast and 2 tablespoons of sugar into my dough.

Hope this helps.