I recommend writing through it, if at all possible. You can always go back and change what's there after the ideas are flowing again. But for severe writer's block, I recommend stepping back, retracing where the plot started, where you are now, and where you want to go. Sometimes it may be as simple as diving deeper into a particular character -- their past, what made them who they are, etc. The answer may be in something that never even makes it into the book. But you know it. You know them. And they tell you.