If I press Ctrl-Alt-Del or click the Task Manager short-cut I added to the Task Bar, Task Manager opens immediately. If I select it from the Start Button's right-click pop-up menu it will not run. Some of the other menu items on that list will not work either, but some do.
The PC is an upgrade from Windows 10. After the upgrade, I ran all the updates listed in Windows Update. Accessing Task Manager from the menu worked after those upgrade/updates but the following day, it did not. I do not believe it failed after a Microsoft Update, but they did Restarts of the PC. A power-off may have triggered this.