Were you on some kind of contraceptives. When you didn't have bleeding since last 6 months did you consult any doctor? There could be multiple reasons for a sudden bleed. Often when you are on contraceptive pills and you suddenly stop you get bleeding. If you are having your thyroid increased there are irregularities in mentrual cycle. When levels become normal you regain period. PCOD is another reason for no bleeding sometimes which can occur suddenly one day. Another possibility could be that you were pregnant and there was a miscarriage.