I am in the same situation.
One of my letter providers, actually my advisor, just have not submitted any letter to the programs. What's worse, he didn't reply any email I sent to him since last Thursday. I have a weekly meeting with him today and I will try to find out what happened to him.
Latest update: when I am finishing this reply, he replies the email I sent to him half an hour ago to make sure the exact time of today's meeting. But nothing else.