You've merely toggled off the display of what is referred to as white space... most likely by inadvertently double-clicking the top or bottom edge of a page while in Print Layout View. It's intended to maximize the editing area by not displaying Headers & Footers.
To restore the view of the document to what you ar accustomed to either double-click the edge once again or go to Word> Preferences - View then restore the check for White space between pages in Print Layout View in the Show Window Elements section of the pane.