What might happen if a well adapted population experienced sudden major changes in their environment?

If a well-adapted population experienced sudden major changes in their environment, they may struggle to adapt quickly, leading to a decrease in population size or even extinction. Those individuals with genetic traits that are not well-suited to the new environment may be at a higher risk of not surviving and reproducing. Over time, the population may evolve new adaptations to better suit the changed environment, but this process can take many generations.