The template file 'normal.dotm' got changed.
With Word closed, navigate to C:\Users\<YOUR_USER_NAME>\AppData\Roaming\Microsoft\Templates
There you will see a file called normal.dotm.
Rename it or move it elsewhere.
Open Word
You should be presented with a blank Word doc.
After you close Word, there should a replacement file called normal.dotm in the above folder.
All done.