I am 26 year old married woman. My period date is missed. Since 10 days. I have done pregnancy test. But it is found negative. What is the reason?

Can't say... Schedule an appointment with your doctor. There are various medical causes for irregular cycles, and only your doctor will know your health history and be able to do an exam.
Pregnancy or..... Pregnancy may be the most common reason for a missed period, and it's possible there is an abnormal pregnancy with lower hormone levels that haven't turned the test positive.. Other reasons include hormone imbalances (polycystic ovary syndrome, thyroid disease, pituitary problems, eating disorders). Sometimes caused by stress, but don't assume that's it. See your doctor.