It's a process I'm familiar with gaming out. For our infrastructure, we need to discuss and update our plan for this from time to time, from 'getting the generator up and running' through to 'accessing credentials when the secret server is not online' and 'configuring network equipment from scratch'.
The electricity people have a name for that: black start (https://en.wikipedia.org/wiki/Black_start). It's something they actively plan for, regularly test, and once in a while, have to use in anger.