Lower a rope down the chimney and into the fireplace. A squirrel will use it climb back up the chimney and out onto the roof. Make sure your fireplace flue is open so that the rope can be lowered far enough. If needed, weigh the rope with a hammer or other similar object. Tie the rope to a stationary part of the chimney or roof so that it doesn't fall into the fireplace when the squirrel climbs it.
Leave the rope for several hours. It may take the squirrel a few attempts to get up the rope once he notices it. Wait for about 24 hours and then call animal control if the squirrel is still stuck. If it doesn't escape it will die in your chimney.
Cap your chimney on the roof. Once the squirrel is gone, cover your chimney with a chimney cap, which will prevent a squirrel from falling in again.
Close all the doors to the room where the fireplace is located. If the squirrel has fallen all the way into the fireplace, it may not be able to get back up and will have to be removed through the house. Closing all the doors will keep it contained to one room, making it easier to get it out.
Cover all light sources with newspaper except for the window or door that you want the squirrel to leave through. This keeps the squirrel from getting confused and offers a clear way to escape. If you are using a window, remove the screen and open the curtains or blinds as far as possible.
Install and chimney cap. This will keep a squirrel from falling into your fireplace in the future.