BlueRose
It all depends on your age. If you are in your 40s, or older, you could be perimenopausal. Before one's periods permanently end, it is quite common to go months without one, then have them start up again.
If you aren't old enough to be perimenopausal, it could be a hormonal imbalance, stress related, a vitamin deficiency or you could have been pregnant and miscarried. Also, eating disorders, such as anorexia, can stop periods.
If you are still in your teens, irregular cycles are common.