If the Earth somehow became tidally locked – in which one hemisphere of the Earth is perpetually facing the Sun while the other remains shrouded in darkness – it would be bad news for life. There would be no seasons, and temperatures on the Sun-facing side would get hot enough to boil water.
Meanwhile, the dark side would become frigid, with the only source of heat being the ocean circulation and winds from the sunny side. The huge difference in temperatures between the two hemispheres would likely create extremely violent winds and copious thunderstorms.
Such a hugely unstable climate would probably mean that all but the most resilient life forms would have to cling to survival along the strip of land between the day and night hemispheres.