It's more common than you think to be late or miss one or two altogether in a year. If they are back to back-you should check with your doctor, but for one missed period I wouldn't worry too much (as long as you know you're not pregnant or ill). It could be a hormonal issue-like thyroid, or just stress if you've missed two in a row, so I wouldn't worry, but you should give you doctor a call. They may give you something to jump start your period and get you back on track.
Charting your periods on a calendar or in a notebook might help you understand your body's unique patterns, so you may want to start that now even though you've missed. The first day of your period is cycle day one-just start and go from there. It seems like a pain to do-but after keeping track for 2 weeks, it's second nature. After a couple of months, you'll be able to tell a lot of things about yourself, and you'll become more in-tune with your body.
I hope all is well with you
Take care!