Lots of different things can cause your period to be late or skip a month; for example, many women skip their periods if they're under a lot of stress, as significant stress can actually stop a woman from ovulating temporarily. Also, illness, depression, grief, a major weight loss/gain, malnutrition, over-exercising, a change in your daily schedule/routine, a change in diet, or a change in medication can cause your period to be late or skip a month or two.
My guess is that one of those things is causing your period to be late.
Don't stress about it. Your period will probably come back next month.
If your period doesn't come back in another month or two, see your doctor.