User interface has narrowed; blank, grey column in right-hand one-third of screen

For some reason, the user interface has narrowed, such that the right-hand one-third of the screen is now grey (blank).  (The reader pane is turned off; I never use it.)  Whatever caused this also changed the layout for writing an email.  Now, when I write an email, the To box goes all the way across the screen, and same with the subject box; the body of the email is under that, rather than the way it used to be with the To box in the left column.  I can't find any way to fix this.