The story I want to tell is about a world where one side of the planet always is sunlittenand the other side is always drenched in blank. The planet once stopped rotating (tangential meeting with a black hole). That is, it makes one rotation a year.
So, when the planet was still having a day-night rythm, there was a variety of life, comparable to life in Earth. The story deals with the problem how to stay alive in this world. Can this be achieved by simply traveling the planet, so an artificial kind of night and day are achieved? Can people (human-like aliens) survive by going underground? Lets rule out spacetravel as this seems too far-fetched to me.
One problem will be how to keep flora and fauna, necessary for food, alive. One side of the planet will become very hot, while the other becomes extremely cold. Maybe the best place is the transient region between dark and light. There will be a huge temperature gradient there though. Will this cause massive storms.
Animals start to die one by one and the living daylights get dimmer and dimmer. What to do?
Science and technology are advanced. There are hyperfast parallel computers, every means of transport and communication, gene regulation knows no borders, biotech is advanced, even the atmosphere can be manipulated. There is not enough energy though to give the Earth a reboot though.
What can they do? Is all life doomed?
There are animals that can live in the dark. Assuming the animals to be similar to here on Earth (I could have used the Earth I guess...): https://animals.mom.com/animals-that-live-in-dark-cool-places-12573467.html
There are bacteria that can live in the hot, cold, and dark. Mamals will die out soon though.
We have the theory of the eyeball planet, as adviced in the comment below. It shows how such a planet is gonna look like. Mercury is such a planet (like our moon with us, tidally locked with the Sun).
Im not asking if life is possible in a tidally locked planet. The mechanism for the dimished rotation is different and it's pretty clear to me that life on such a planet is difficult, if not impossible. When nothing is done. I'm asking if the alien people can find ways to survive without leaving their home planet. What else can I add?