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Short answer: Talk to a doctor if you're seriously worried.

Slightly longer answer: There are plenty of reasons, serious and not, why a woman's menstrual cycles might be irregular.

i suspect you probably know the answer to this already, but anyway...
it all depends on your 'usual' cycle, if you are always right on time then obviously its time to get it checked out. if you are often irregular then of course it depends on how irregular, has this happened before etc.
it does happen that you can miss an entire cycle or be very very late due to stress, hormonal imbalance, major changes in diet ( i mean extreme like anorexia ), cysts or blocked ovaries can create changes in your cycle....the list of possibilities is long, but the fact that you question it here seems to me to point to your concern of possibly being pregnant.
in that case, get a test and find out. its possible to tell very early on now.

Speaking as a man (i.e. what use am I, etc.) I have known of many occasions of this kind of lateness. Unless you have always been extremely regular for a long time, and you haven't had any illnesses this last month, it's too soon to worry.