What Causes Delayed Periods?

Good morning doctor, I will would like to ask you about my condition, I am now 35 years old, Married and got 3 children. My problem is my last menstruation was on the 15 December 2014, and until today I haven't got any menstruation, I did the pregnancy test and the result is negative,I am very worried because I want my menstruation to come out as last month.Will it be possible to take Gynaecosid table would help solve my health problem, if so what doses will I be taking...Thank you in advance doctor for your kind respond