I Went For My Routine Visit Ay 16 Weeks,. Doc

I went for my routine visit ay 16 weeks,. Doc was unable to hear the HB, went for ultrasound...and wastold there is no cardiac acitivity since13 weeks. However my first nt scan dual scan...all blood test reports were normal at 12 weeks. I didnot have any cramping..bleeding any symptoms. In the second month i was having the problem of spotting but it was fine later on. Why did the heartbeat stopped?