It is highly likely that you did not do the test correctly. You need to follow the instructions on the packet to get a correct reading. It may just be that you have taken the test far too early.
There are also other reasons for not starting a period on time. So like mitchelljj says, you need to wait another week or 2 and do the test again following the instructions, or make an appointment to see your doctor.
Sometimes the at home test do not detect the hormon to give a possitive reading. Some woman have to get the blood work test to know for sure. Wait another week and take a different test if it turns out negative then get ur obgyn to do the blood test. Dont stress cause this could delay ur period. Hope this helps!