Snickerdoodles normally puff up during baking, then fall a bit when they come out of the oven. If they didn't crack, it means they didn't rise, probably due to your baking soda being old. Test it by sprinkling a bit in a glass of vinegar. If it doesn't fizz, it's time to replace it. It's also possible that your oven didn't get hot enough — check the temperature with an oven thermometer.