What happens when your babys umbilical cord detaches while in the whom?

I actually knew someone that happened to... it was really sad. Her little boy died. Since that their sole source of survival, if it detaches, I guess they suffocate etc... ugh that is a horrible thought. If you find out about it soon enough, you would obviously have an emergency C-Section, but not sure how quickly you would find out if it happens while you are at home and baby is not being monitored. Oh my gosh that just makes my stomach turn thinking about it!