Sudden delay of menstruation for 20 days can be caused by a variety of conditions, as follows: 1. Pregnancy: If you have sex, you first need to rule out the possibility of pregnancy, which will generally lead to the absence of menstruation after pregnancy. 2. Physical factors: Excessive mental tension, emotional changes or cold stimulation, excessive weight loss, excessive exercise may lead to a delay of menstruation for 20 days. 3. Suffering from endocrine diseases: Polycystic ovary syndrome, high prolactinemia, etc., may also lead to sudden delay of menstruation. In this case, you can use progesterone for treatment and then have a blood test for female hormone 6 on the 3rd-5th day of menstruation to rule out endocrine diseases.