A family of Microsoft word processing software products for creating web, email, and print documents.
Your document got corrupted. Most likely causes are a faulty math equation or an index field. There are other errors, but those two are the most likely. The good news is, it can be fixed with minimal data loss. The bad news is, you need some (open) xml knowledge.
A Word 2007 docx file is nothing more than a zip package. If you change the extension from .docx to .zip, any zip archiver will be able to open it. In the archive you will find a couple of xml files. The main document is commonly (but not necessarely) stored in the /word/document.xml. If you would check it out, you will notice that the xml tags are not correctly balanced. Fix that, and your document should be fixed.
If you don't have the knowledge and the document isn't all that large and not confidential, upload it somewhere and post back with a link. That way, someone might be able to help you by fixing your document for you.