I actually have encountered your scenario frequently. Surprise power outage isn’t an uncommon thing here. So I don’t even have to think about it like you say. I just have to tell you what I usually do in the situation.
Good thing I’m always either with my phone or my computer so I still have a source of light with me. Usually when this happens I reach for my phone and just stay still in my place. I don’t panic because I know my eyes will adjust to the dark and soon I will be able to have a bit of vision.
My dad is the one that has control of the generator so I can wait for him to power it on. But he doesn’t always do that because the generator costs lots of fuel and we don’t always want to waste the power. So if he doesn’t use the generator he will just grab out big light. Often my parents would then call us to gather in a place. And that is when I have my phone ready. The gathering is meant to get us to stay in one place and wait out the outage, hopefully. Sometimes dad just hands us the lights we have a lot of lights in the house and I just go back to my room and wait with the light that I try to use sparingly.
I have lived through so many power outages that I essentially stop being surprised about it. When it happens I’m just like “oh not again huh?” There is just no fear at all.