Microsoft Word stops accepting non-text keys (a.k.a. keyboard stops working in Microsoft Word)

Problem:  Word stops accepting non-ASCII keys.  Everything works using mouse shortcuts, but keyboard equivalents do not work.

For example, you can type "normal" text.  But arrow keys, function keys, and keyboard shortcuts such as Ctrl-X for copy, Ctrl-V for paste, do not work.  Home and End keys do not work.  Function keys like F7 for spell check do not work.  Number pad arrows do not work.

Possible solution:  Using the mouse, try starting the spell checker (Review / Spelling & Grammar.). 
For whatever reason, this unlocked all of my special keys again in Word.