A family of Microsoft word processing software products for creating web, email, and print documents.
It's possible there's a fault with your Office installation installation. Have you tried repairing it (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change)?
It's also possible the document has acquired some for of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.