Settings or problems with your web browser may cause the help option in Office 2013 or Office 2016 to stop working. To fix the problem, try the following:
Install the latest version of your browser
Install the latest version of your default browser to avoid any compatibility issues :
Troubleshoot your browser
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For Internet Explorer, you can:
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For Chrome, follow the steps provided in their troubleshooter.
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For Firefox, follow the troubleshooting methods described in their article.