'i am 15 days late period but negative pregnancy test. Don''t know what to do now? '

Possible reasons. Reasons for missed/late periods: low body weight, obesity, marked weight ^ or v, over-exercise, endurance athletics, breast-feeding, ^ stress, illness, eating disorders (bulimia or anorexia), anovulatory cycle (no egg released), travel, hormone irregularities, drug use, meds (i.e., birth control) or medical problems (i.e. PCOS). #1 reason is pregnancy.
See your doctor. If you did the test correctly and it was negative at 15 days after the due date of your period, you are not pregnant. To make sure, you may wish to see your doctor for a blood test and/or ultrasound test.