The problem has been caused by a Windows 10 update unleashed late last week, namely Cumulative Update KB3124200, which Microsoft noted made some tweaks and offered "improved functionality" for the OS (yes, a typically vague patch description).
Except it didn't improve Microsoft Word's functionality, and as The Register reports, it caused Word's Normal.dotm template to be renamed, then recreated and reset to its default settings (losing the user any customisation and macros saved in the template).
As 'Rob L', Group Engineering Microsoft Word, posted on the Answers.microsoft.com thread regarding this issue, what's happening is that the existing Normal.dotm is being renamed to Normal.dotm.old during startup.
So the good news is that this is simple to cure, as all the settings are still saved in this file, which is easy enough to recover from the Templates directory. If you're affected by this problem, check out the full instructions on the forum here.
Redmond suffered a substantial number of complaints over missteps with the major November update for Windows 10, plus it has also recently been under fire over issues with its new Surface devices, and a raft of firmware updates which have been pushed out shortly after launch – with some arguing that the company simply isn't testing this stuff enough before it is loosed on the user base.