What will happen to mr and mrs de winter after they left Manderley?

We'll never know; the story ends as they approach Manderley..... much to Mrs. De Winter's surprise.

if you go back and read ch.s 1 & 2 it tells you more. it talks about how they are at a little hotel and how they don't ever talk about manderley (but always think about it). but other than that it doesn't tell you much and we never figure out the whole story.