What Causes Delayed Periods After Taking Ipill?

hey doctor..i m 20 years old i recently had unsafe sex with ma boyfriend..but latter on i did take an ipill on time..now m nt having ma periods since a long time its almost 45 days but when i had done a pregnancy test at home the result turned out to be negative i am bit tensed can u suggest me what to do..and which pill to take to get the periods.?