Luckily, I've not had much of that, yet, but I know that can happen. Sometimes when I'm a little stuck as to where the story is going next, I step away for a bit, spend time with my family or maybe take a nap. My characters seem to invade my mind when I relax and try to go to sleep whether it be trying to take a nap or going to bed for the night. Many times they will give me a plot twist or tell me a big place the story will go. Then when I get up, I have things I need to write down and start writing again.