I have been missing periods from two to six months what might be the reason?

Endocrine problem: Once you have ruled out pregnancy your provider will evaluate your endocrine system. The hormones that regulate your menstrual cycle fall under this system as do the hormones that regulate your thyroid. There are some basic tests to start as well as a good history of menstrual cycle and contraception use from start of cycle to present. If the hormones are off it can affect cycle.

Hormone imbalance: A hormone imbalance that causes you not to ovulate. You need a full work up to be sure there is nothing else wrong. It is not safe to go longer than 3 months without a period. See your doctor

Missing peroids: I assume you were having normal peroids before. Something has chaned. More likely a hormone change. If you are healthy then there can be simple things : birth contol pills can do this, excessive weight loss or weight gain , and excessive exercise. Emotional upsets and worry can cause hormonal imbalance. Make sure you have a normal exam. To make sure of no ovary problems or thyroid problems.