I also suggest you close Word, then reboot Mac in safe mode, after that, open the document again. This helps you check if issue only happens when you use Word desktop app. If the document is saved on your Mac, you may also try to make a copy of it then upload the copy to OneDrive, see if you can open the copy via Word on the web. People have gotten this somewhat randomly with older Word versions, so I suspect the latter.
The Mac Word plugin hasn't changed in months, so this would have to be a change in a Word 2016 update or something else on your computer.