The hymen is a thin tissue that covers the opening of the vagina. It can sometimes be visible in either a moon shape or sometimes a circle at the opening. It may be difficult to see if you are doing some self-inspection depending on its size. Intercourse and other sexual activity are not the only ways that the hymen can be broken. Sometimes physical activity and tampon use can stretch the hymen out of place.
This is not intended to be medical advice. Everybody is different so please make sure to consult your physician if you're having issues. Do not delay or refrain from seeking professional medical advice from your physician because of something you have read on this site.