What happens when you send an iMessage to someone who has an iPhone, the messages show that it was delivered, but then it shows nothing?

I was sending messages to my friend through my iPhone, it showed that it was "delivered" each time, and then out of nowhere I sent a few more and it showed nothing? I can't tell whether the remaining few messages were sent or not. Does that mean I was blocked? Is there a problem with the cellular data on my end or on my friend's end? Was my friend's phone not on? What could it be and how can I tell?

Ask the person to whom you sent the messages when you stopped getting delivery confirmation.