What Causes A Delayed Menstrual Period?

i am aparna (26,single). i always had a normal period, but suddenly this time it is 1 week late. it has never happened before. my family history is also clear no one has any type of problem ,such as ovarien cancer etc. so wat can be the probable reason for this. otherwise i am normal , even i dont have any pain also in abdominal areas or other place..