It's possible the document has acquired some form 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. Similarly, corrupt tables can often be 'repaired' by converting the tables to text and back again or by saving the document in RTF format, closing the document then re-opening it and re-saving in the doc(x) format.

It's also possible there's a problem with your Office installation. Have you tried repairing Office (via Start > Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change)?

Beyond that, without actually seeing the problem document it can be difficult for anyone to diagnose the issue. You can attach a document to a post (delete anything sensitive) via the paperclip symbol on the 'Go Advanced' tab at the bottom of this screen.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]