I accidentally changed a User folder into a a desktop. Is there a way to restore original user folder?

That error was shown because the your user name folder was renamed to Desktop. To fix that error, follow these steps:

  1. Rename the new Desktop folder to your user name. Let assume user name is Andrew. For example, the renaming will be from C:\Users\Desktop to C:\Users\Andrew.
  2. Check if a folder exists as C:\Users\Andrew\Desktop. If that folder doesn't exist create a Desktop folder in C:\Users\Andrew directory.
  3. Then try to 'Restore Default' as usual. See this article for further reading.