Hey I'm 21 years old.For like 13 months now I haven't got my mensturation,can u please say why?

What has your doctor said about it? There is likely something hormonal going on. Could be a number of possibilities such as PCOS, elevated prolactin, etc. Are you on any sort of medication, including birth control? Do you have any chronic illness(es)? What is your general health like?

If you haven't seen your gyn about it, and had some testing done, that would be the first thing to do.