What Causes Delay In Menstruation Other Than Pregnancy?

Hello Doctor, my name is iffat . My last period was on July 04, 2010. Since then i have not got my periods. Also, i did my complete check with the gynae on July 11th with the blood test and it showed normal ovaries and neg. preg. test. She told me to take duphaston after 25th of july after doing hpt. I have still not taken the tablts. Iam just wondering what must be the reason for this kind of dealy. I would be more than happy if i get my periods naturally. Please help??