You can completely miss periods and still not be pregnant. You need to identify how late your period is. If it is more that two weeks late then first of all take a home pregnancy test to rule out pregnancy. Then I would suggest a chat with your doctor if the period is more than a month late.
There are many reasons for missed or late period including hormone or thyroid problems, being under or over weight, exercising or dieting to extreme levels. Stress and polyps are also other reasons too.
So depending how late it is, you may need to see your family doctor. If this is an ongoing problem then going onto the birth control pill can sometimes 'kick start' your cycle.
Does this help?