If you use Word 2016, the latest version of Microsoft's word processing app, and install the company's most recent Windows 10 patch, the update wipes out your "normal" template. That little file, named normal.dotm, is important because it contains all of your macros, autotext blocks, autocorrect entries, styles and more customization options. If that normal.dotm file gets nuked, all of your tweaks and customizations are erased, and you need to recreate them, which is a major pain for folks who use heavily customized templates. Word still works, but the software reverts to its default settings.
A Microsoft engineer who monitors the official Office 2016 user forum confirmed the problem and suggested a solution. The good news is you can recover your customizations. However, it's a fairly complex process. I won't detail the steps here, but if you hit the link above you'll find more information on the recovery.
"This was not an intentional change and we (the Word product team) are working to understand both the cause as well as what steps customers can take to either avoid or recover from this," the Microsoft employee wrote. As of Tuesday evening, no official fix for the update has been released.
Windows 10 often automatically updates itself when Microsoft pushes out patches. To date, the company released one very large update, and a series of smaller patches. The latest patch, called "Windows 10 Cumulative Update KB3124200," renames the Normal.dotm template, which causes the problem when users restart Word, according to Microsoft.
I've seen a few reports that indicated older versions of Word are also affected by the upgrade glitch, but so far Microsoft only confirmed the problem with Word 2016.
If you use any version of Word on a Windows 10 PC, I suggest making a backup of your normal.dotm template before you install any new Windows software updates. You can find your template by navigating to Start > Run, and then pasting %appdata%\Microsoft\Templates into the entry field. Next click OK, and you'll see the templates.
If you update Windows 10 and it renames your normal template file, replace it with the backup, and you'll be back in business.