I do use Emacs for years, and in particular the Cygwin Emacs version (under Windows 10) for months.
Since 2 days, M-x combo key is not working anymore, well ESC x but I don't want to be forced to use Emacs à la Vi ;-)
First, I thought that the keyboard of my laptop could have a trouble, but the same effect is observed with an external keyboard connected by USB.
Second, I though it could be due to some changes I did in my own Emacs init file, or changes inherited from MELPA. To test that, I came back into time with Git, but that wasn't it: problem still observed; to test for MELPA packages, I used a minimal Emacs (emacs -q) and, once again, the problem still persists.
(And ESC x displays that M-x, translated from <escape> x, runs the command
execute-extended-command -- which is expected.)
Finally, what's weird, is that the key x alone is working, and so does C-x. On the other hand, M (meta) is working: M-c, M-w do work like expected.
So, to sum up, only the particular combination M-x is not working, and I really have no idea why.
Any smart test I could do to debug this (on Win 10)?
To sum up the answer in the comments:
If your key combination does not do what you expect - use C-h k to find out if it's a wrong mapping inside emacs, or something is grabbing this combination before emacs has a chance. If C-h k doesn't do anything in response to Alt-X - then you can be certain emacs doesn't see it at all.
If you want to confirm a key combination has been hijacked try to use some other application with it. Alt-X isn't used by regular Windows applications, but if you happen to have anything else that can have customized key bindings then try to map Alt-X to anything inside it. Eclipse, IntelliJ, Visual Studio, gvim. But seeing what happens with C-h k is a sufficient proof.
To find out what is stealing your Alt-X check what was installed or reconfigured lately. Disable startup programs if you can't recall. Additionally, from my experience a couple of nasty offenders: Intel video driver, and Windows language bar. (Not with Alt-X, with other key combinations.) Windows language bar is especially annoying when it steals C-S-) which I use for slurp.
Last thing. If you are running in a VM, your key combination may be stolen at either host or guest level (or both, as I've seen with the language bar.) So you may need to look around in both places.