Two possibilities immediately come to mind:
1. A faulty Office 2013 installation;
2. A corrupt Word document with a form of corruption Word 2013 is sensitive to.

For the first, you could try repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office 2013 > Change > Repair).

For the second, 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]