Eu achei essas instruções úteis, você pode acessar os estilos e "apagá-los" para redefini-los aqui: Abaixo do arquivo - > Opções - > Suplementos - > Gerenciar: [Modelos ^] [Ir ...] - > [Organizador ...]
OR
Eu também mudei o DESIGN do documento, que parece ter limpado o problema Heading Styles para mim.
Fonte: Q: Estilos Corrompidos Como faço para redefini-los?
You can delete the heading styles in the document and apply new styles by going into:
Tools, Templates and Add-Ins and then click Organizer. The styles in your document will be indicated on the left side of the Organizer dialog box.
Select all the heading styles and press "delete" a dialog box will appeared asked if you want to delete the styles and click on 'yes.' Word will take you through a series of "Cannot delete Heading 1-9." Each time click the 'ok' button.
The heading styles has been reset and thus any formatting you've done to them in the document will either disappear and you will be left with just the text.
Now you're ready to drop in fresh heading styles. To the style box on the right, scroll down to the heading styles and select the headings 1-9 and then in the middle of the dialog box, click "copy." (Notice how the "Copy" button has the arrows pointed towards your document Style gallery.) The heading styles has been copied over.
Just go through the steps of modifying the heading styles for your document and then reapplied.
Hope this helped. :)
