I have not had my period in over 10 months. i am 18 and not sexually active and have always had an irregular cycle, but it has never been more than a 3 month gap. i was wondering what some possible causes are and how serious it is and what i can do?

A syndrome called "polycystic ovarian syndrome (or condition)" or PCOS can result in cessation of ovulation, which will postpone true menstruation, and caused non-ovulatory episodes of bleeding at very irregular intervals, secondary to sustained production of estrogen causing thickening of the endometrial lining which when breaking down , can cause sustained irregular hemorrhage.