What happens if you don't have your period for 1 month and you're 17?

It depends on whether or not you are currently sexually active. If you're not, the chances are that you've just missed it because of fluctuating hormonal levels or stress. It's especially common to be irregular in the first few years of having your period, because your body is still trying to figure itself out. If you are sexually active, however, you might want to look into taking a pregnancy test. It's unlikely that you are pregnant, but it might be useful jus so that you can rule it out as the cause. Again it's most likely fluctuating hormones, but it might be worth checking out anyway.