What Causes Stoppage In The Menstrual Cycle?

Hi, may I answer your health queries right now ? Please type your query here...hi doc im 27 yrs old female i had got my last periods on 10th feb 2011 aftr tht till date i hvnt got periods so i did two tests one with preg card at home and one at doctors pathology for pregancy but both tests came negative nw im worried and tensed plz help me wd this asap