A family of Microsoft word processing software products for creating web, email, and print documents.
Your description indicates that the Built-in Building Blocks template (you may or you may not see the .dotx extension), which stores custom header/footer formats, have been damaged. You can delete the template, with Word closed.
Do the following: Exit Word if it is currently running. In File Explorer, go to %appdata%\Microsoft\Document Building Blocks. If you have a US English installation, open the 1033 subfolder and then the 16 subfolder. You will find two templates—Building Blocks.dotx and Built-in Building Blocks.dotx—and you should delete the latter one. When you are done, restart Word.