What Causes Delayed Periods?

Hi, I have missed my period for 3 days now, my regular cycle is 27 days, today would be day 30. Though, I have never had sexual intercourse. I have had sexual contact within the past month, though there was so penetration, and no male ejaculation. What should I do? Do I wait for a few days to do anything? How long should I wait? I have no pregnancy symptoms at all.