If you open "Manage app execution aliases", do you see the entries for Python? I've heard that they sometimes switch themselves off during updates, but nobody seems to know why. Even if not, disabling and re-enabling them may help.

The executable in Program Files cannot be launched except in a few oddly specific circumstances. I've spoken to the team responsible and they consider this part of the feature, so the execution aliases mentioned above are the way to go (these live in your own AppData folder - check sys.executable once you get it going again).