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.

in this case, though, I'm not sure that will solve a Style-based corruption. To fix that, you may need to:
• save the document in RTF format;
• open it in WordPad;
• make an edit in Wordpad (e.g. insert a space);
• save the document & exit Wordpad;
• re-open in Word.
This process should remove all Style data from the document, though the formatting will be retained. Assuming the problem isn't in the document template, you should now be able to re-attach the Styles.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]