Word uses templates. There is a 'default' template that every new document opens with. I can't say why your default template has changed recently but if you want to change it back you have to edit this template.
It should be in your 'templates' folder and named 'normal.dot' or 'normal.dotm'
1. Open Word
2. Click on the Microsoft Office button
3. Click on Open
4. Look at the top of the pane that opens and you will see a folder called Templates
5. Click on the folder and see normal.dotm in the list, double click in it to open
6. Make the changes then Save
The next new document should open with your new defaults.