I have no clue about the McDonald's ice cream machine but the one I used to have to use when I worked at Burger King could have various issues, for instance the most common reason it went down was it getting low on mix and someone not refilling it, if that would happen for about 30+ min it would freeze the thing solid and at that point you have to turn it off for hours, and you still probably have to tear it apart and run hot water through it which would make a huge mess just to get it in unfroze. Worst part is I was a closer, and if the people on the shift before me weren't keeping up with it, I could come in to it being frozen... I hated that thing