Say you've got a hypothetical human-colonized planet planet roughly the size and composition of Earth orbiting around an M-class star a third the size of the sun at 0.2 AUs. Due to the proximity, this planet is tidally-locked. There's a ~518 mile wide terminator of moderate, human-habitable temperature and the other ~96% of the planet is either boiling ocean/desert or tundra/ice. A tremendous storm at the substellar point blows precipitation and heat to the rest of the planet. The population of this planet is ~1 billion, the vast majority living in the terminator.
And then some astronomic-sized magic event
such as a comet (sure let's go with that) knocks its rotation speed out of the normal (assume it doesn't mess with the orbit somehow, only the rotation) and the planet starts rotating out of sync with its orbit--the tidal-lock is broken. One study says this may have happened with Mercury, although I'm interested in the effects on a planet with a human and animal population: http://www.space.com/13889-mercury-spin-asteroid-collision-tidal-locking.html
(Magic and iffy science acceptable, this is for a story where magic is a thing.)
Edit: Took out the comet thing which was just intended to be an example.