Well the obvious first question is whether or not you engaged in any potentially risky behavior this month that might have led to pregnancy. Did you engage in sexual activity (protected or unprotected)? Did you experience a failure in your method of birth control?
Young women often have irregular cycles (especially within the first 5 years of menstruation). It could be that your partner's cycle is just irregular or that she simply has a longer cycle than you expect (not everyone has a cycle that lasts 28 or 30 days, ya know). Other things can also cause delays in the appearance of one's period (we've got a whole article about that - you should check it out).
If test came up negative, then she should wait a week and test again (if her period still doesn't show up) to try to verify the results of the first test. If she is concerned about taking another hpt, she can go to her doctor, local clinic, or Planned Parenthood location for the test.
Scarleteen Sexpert (and Labia Lady)