Hi, I Had My Last Period In Dec 2015 And

hi, i had my last period in Dec 2015 and after that still i havn't got my periods, i am 40 years old and i have 28 days periods cycle. this is 3rd time of my irregular periods first in july 2015 came after 36 days then 41 days and now its almost more than 75 days delayed. kindly suggest.