What Causes Missing Periods And Negative Pregnancy Tests?

hello i had protected sex with my partner 5 to 6 times in last 2 months. my periods last happened last on 10th jan. i havnt got my menses since then.
i had hd checked with the pregnency check kit and the result was negative. i had checked in the evening and not morning.
please suggest what should i do to get the menses start. i am worried plz suggest something.....