If your Snickerdoodles came out hard or crunchy, they are likely over-baked. When you take them out, they should be slightly puffed and no longer gooey in the center, but not browned. They will look slightly underdone but will continue to cook on the baking sheet as they cool. It could also mean that too much flour was added. Weighing your flour is the best way to ensure your cookies turn out perfectly every time.