Possibly the font or the document itself has become corrupt. If other documents using the same font are affected, delete then reinstall the font.

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.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]