You might repairing the Office installation (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair), then checking whether the problem is resolved.

It's also possible you have a faulty third-party addin. If running Word in safe mode (i.e. hold down the Crtl key while starting Word) resolves the problem, a faulty third-party addin is the most likely cause. In that case, re-start Word, disable all third-party addins, then progressively re-enable them until the culprit can be identified by its problematic behaviour. Uninstall (and, perhaps, reinstall a compatible version of) the addin.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]