Rule nr. 1: Don’t save your work in a file format foreign to the software you work with and make back-ups regularly. Save in odt format, only save as docx when you can’t avoid it, that’s when somebody asks for a copy in that format. For such cases you almost should have a copy of MSO.
Something went wrong when the xml was written to disk. All odt and docx files are saved as a zip archive, the actual content sits in a file called content.xml (odt) and, IIRC, document.xml (docx). Change the extension to .zip, open the archive, extract the document.xml, open with a plain text editor (like Notepad in Windows, but definitely not Writer, scroll to the offending location, see if you can repair the error, save the edited file, put it back in the zip archive (better still: in a copy of the zip archive), change the extension back, try to open the file.
Sign up at http://forum.openoffice.org/en/forum/ - a real forum and ask your question there if this didn’t work.