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!
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.