Some reason of delayed periods are
2.short illness like fever
etc. or even a longer illness can delay your period
3.A major change in your weight either by gaining a lot or losing a lot of weight
Your HPT is negative and repeat this test after 5 days and if it still is negative then please contact a local gynecologist to find out the cause of delayed periods. Any of the above mentioned apart from pregnancy (If Second HPT also negative) could be the cause and treatment depends on cause.