After checking the web, and eliminating the various issues they described (e.g., word wrap on, macros off, foreign keyboard setting, date/time not set correctly, etc., none of which applied to me), I started in "Safe Mode" (or what passes for such in Win 10), and had no problems!
I then went back into regular Windows, and tried a controlled test bringing up one program at a time. The only one that caused the problem was Facebook Gameroom (fortunately, it was just the second app I ran, so I didn't bother running any others). I clicked the 'X' to close it, but ALT-ENTER would still not work. I then discovered that it was still running in background, so I "exited" it from the taskbar's up-arrow widget (i.e., where some "hidden" apps, such as the anti-virus program, are running (see photo, below)), tried ALT-ENTER in Excel, and it worked just fine! I repeated the test, and it did exactly the same thing!
So, for whatever reason, Gameroom is preventing the ALT key from working in Excel, Word, and, possibly, other apps, as well. I sent them a bug report; they're pretty good about fixing things like this, so, hopefully, an updated version of Gameroom will be coming our way, soon! And, no, I didn't make the connection between the failure and installing Gameroom, because I didn't do one right after the other; I don't recall when it first happened, but, it might have occurred after a Gameroom update, not the initial installation.
Of course, there may be other apps/programs that may cause such problems, so, you may need to do some controlled testing of your own. Start with the most common thing that you run (e.g., web browser), then the next (e.g., text editor), etc., making sure to test ALT-Enter (or whatever) in your affected software (in my case, Excel and Word) before you run the app being tested, while it's running (do nothing else with that test app), and after immediately closing the test app. If it works before and after, but not while, the test app is running, then you found your culprit! I was able to keep Excel up and running through the entire test; the failure was immediate, but, in some cases, you may need to restart your affected app after bringing up and knocking down each test app, to be certain that it's "clear" of any negative affects.
|Click the "up" arrow (shown above my overlapping red line),|
then right-click Gameroom (circled),
then click 'Exit'