A tidally locked planet does not rotate in relation to its sun. One side is always day, one, always night. This is caused by tidal forces pulling all planets towards this same equilibrium, so it’s completely stable once it does occur…a tidally locked planet at an earthlike distance from the sun would be scorching heat on one side, freezing ice on the other.
What about at different distances? Is there a band of orbital distance where the night side of a tidally locked planet is warm enough for liquid water? Or one far away enough that the day side can have oceans?
Given the crazy temperature differences you get on Mercury I don’t imagine the dark side of a tidally-locked planet would ever be habitable, though maybe a proper atmosphere would change that. There would be a habitable ring around the planet that’s partly-lit; seems like the kind of thing explored in sci-fi a lot (though the only story I can think of is The Last Day by QI/NSTAAF’s Andy Hunter Murray).
Interesting note: Mercury isn’t tidally locked, but it sure looks so. Its average day is slightly longer than its year. Learned that a few months ago
I learned it when fact-checking my original comment!