The power off and the plug off. I was wondering how this works, it might have an inner battery that it works for the reset.
I had the same problem, tried to open my selphy cp1200 after some months of its last use and after pressing the power on button it froze to the initial canon screen. Searched around the internet and found how to reset the printer and i decided to try it. So in order to reset it, you unplug it (if you use the battery i suppose you must remove it from the printer), then you press 5 times the power button (give it 1 or 2 seconds pressed and 1-2 seconds between the 5 times pressing it), then wait for 2 minutes. Then plug it to the power and press power on. Mine worked fine after that (at least for that time, don't know how it will respond the next time, hope it goes well!). I hope this will help!