Help. Text won’t flow from one line to another. The words are there when I delete forwards but won’t move over to the next line. What have I inadvertently done? 
Perhaps you have accidentally switched to Page View mode and are using a window width that is sufficiently narrow to obscure the sides of the editor? See if View/Page View/Hide Page View is available as a menu command, and trigger it if so (if it says “Show Page View”, then this isn’t the problem).