How do I reset the desktop zoom level to its default in Windows?

If you mean the icon sizes just right click on the Desktop, go to View, and change to medium.

If you mean your resolution got screwed up, right click on the Desktop and go into your screen resolution settings.

If you accidentally opened the magnifier utility, you can CTRL+ALT+DEL and you should be able to close the interface.

I also thought that CTRL + 0 wasn't working to reset to 100% zoom ... but it is only when I use the 0 key on the extended keyboard that it fails, using the 0 key one main keyboard (above P) that it resets to 100% ... I'll just need to get into the habit of using the main number keys instead of the small numeric keypad that I am so used to.