12 week appt and my baby is dead

What I thought would be the big announcement day turned out to be devastating .. My baby has passed away for unknow reasons .. I'm confused n hurt not understanding what happened. No bleeding or cramping but no sign of baby .. At my 8 week appt the heartbeat was strong and the baby looked great according to dr.. Someone please help .. I'm suppose to have dnc tomorrow .. Why did this happen ?? 💔