It helps if you have an outline, hopefully a very detailed outline, because that’ll tell you what you should be writing next.

Then, if your brain still refuses to work, look over the outline and find some part of the story that you can write. That you brain will work on. It doesn’t have to be what comes next because, obviously, that part isn’t coming to you. But if you write whatever part of the story you are able to write at the moment, then at some time in the future, you’ll be able to connect the parts you have written. And, surprise of surprises, you’ll have written the part that froze your brain. And you’ll be done.