When Characters Refuse to Talk

I had a good day writing my new book today. But I've had many bad days. How do I define a bad day? That's a day when I can't make my characters talk to me. They go into hiding and refuse to come out and play.

On those days, it's best if I sit down and define plot points, because Characters don't talk when they run out of conflict . If characters are a novel's life blood, then conflict is the muscle that moves them. So if I lack sufficient problems confronting my people, they stop talking. And I have a bad writing day.