a real problem for you. there are ways to give the squirrel a way out such as hanging a rope into the chimney. ropes will provide a tough and rough surface for the squirrel to climb. they may not leave right away so leave it for several hours and return to check to see if the squirrels left. if not then leave for a while longer or try another method.
sometimes squirrels are not stuck in your chimney but just choose to live there and raise their young. you may not be using your chimney much so it can be easy for a squirrel to live in there. this may require 