Your Word document has probably become corrupt. Please see the following Microsoft MVP article on why documents become corrupt: http://www.mvps.org/word/FAQs/AppErrors/CorruptDoc.htm
To recover your document please try either of the following:
Select File + Save As Web page, quit Word, reopen the htm file and save it back in Word format – that usually (but not always) gets rid of corruptions. The HTML/XML format forces Word to completely re-create the internal structure of the document, either fixing or discarding the corrupt bits when it does. Best of all, in the case of Word 2000 and above, almost all of the formatting and page layout is preserved.
Please note: to preserve your formatting, you must select the plain Save As "Web Page" option, not the Save As Web Page (Filtered). If you use the Filtered option, you remove from the document all the formatting that an HTML browser cannot interpret: for example, page numbers and headers and footers! (from the MVP document)
Copy and paste your document section by section to a new blank Word document. Format the bibliography as you go. When you come to a section that displays the same problem you know that this is where the corruption is. Try deleting some of this section until the problem goes away. You may need to manually replace some of the corrupt section.
If you are still experiencing problems with this please email a copy of the document to <email@example.com> including a description of the specific problems you are experiencing.
For more information on working with Microsoft Word please consult the Microsoft Word MVP FAQ site http://www.mvps.org/word/FAQs/