if it is neg take another in a week or two if you dont want to wait go to the drs and get a blood test they are faster and far more accurate.
When did you do the test? If you took it say a week ago you could do another, sometimes the level of hormone isn't high enough to show positive. It should definitely show now you are 10 days late.
Have you had any stress this month or started any medication? There is lots of advice on this site about why our periods can be delayed.