1. Make sure the window is maximized—that's caught me before!

2. If you don't have much to lose—custom styles, macros—with Word closed, delete Normal.dotx [and Normal.dotm if you have it]. Word will recreate Normal.dotx with default values next time you open Word.

3. Disable the scrollbar, close & reopen Word, and re-enable it—sometimes settings need a friendly kick in the tickbox.