If the Quick Styles Gallery is no longer showing up or is behaving in a unfavorable fashion, you can force Word to rebuild the file. This will place the default content into the gallery and will not include any modifications you may have made to the content. Once the rebuild has been completed, your Quick Styles Gallery should function normally.
1. Make sure that Word is closed.
2. Open the Run dialog box (use WinKey+R).
3. Input %appdata%\Microsoft\.
4. Click OK.
5. Open the Document Building Blocks directory.
6. Open the 1033 directory.
7. Delete Building Blocks.dotx.
8. Close the window.
9. Open Word.
10. On a blank page, go to the Ribbon and select any gallery dropdown (for example: go to the Insert tab and select Header).
11. Once you see the gallery appear, the file has been recreated and should now work properly.