We've all been there. You're installing a new major system for a client and you're finishing up with installing all required updates and this happens.
Image taken from petur.eu
Or, more specifically, this is the one I am seeing right now (sorry for the German):
The message is pretty clear. Do not power off or unplug your machine!
But I have been looking at this screen for 2 hours now and I begin to suspect that whatever is hidden behind it, failed.
So, what is the right thing to do in this case?