There are many reasons for a late period that may not indicate pregnancy. You may just be having a longer cycle, a later ovulation, an anovulatory cycle (for some reason you didn't ovulate this cycle), you may have a hormone imbalance, thyroid or pituitary dysfunction, or stress may be causing the hypothalamus to prevent you from ovulating this month.
You also may be early pregnant and you may be getting a false negative because you ovulated later in the cycle than you anticipated.
You should retest in one week, and if the pregnancy test is still negative, and you are usually not irregular, you should contact your physician for further guidance and advice.