If you had periods on 20/4/2016, and if the test is Negative , then you are NOT pregnant.
you have done the test carefully, and double checked it
You have not indulged in sexual activity in past month
sometimes the tests are false negative and it would be better to check them again using a early morning sample of urine and doing it in a reputed lab.
If you are second test also comes negative, it means you are NOT pregnant and then you may either wait for the menses or meet a Gynecologist to take medicines to bring menses.
If you are in regular sexual activity, I would suggest to use a double protection of
condoms- which prevent pregnancy and sexually Transmitted diseases
and Oral contraceptive pills to prevent pregnancy
Pregnancy is not there. Thete sre other reasons of not having period in time. Please consult gynecologist and take drugs for starting periods. Please read-https://drsujoydasgupta.needstreet.com/post/8-ways-to-avoid-irregular-menstruation/2339