Try running the cpu on the system for awhile watching the temps (don't let it get too hot) and allow it to warm up. Turn the system off, then unfasten any mounts holding the cooler to the cpu and while it's still mounted in the motherboard gently try twisting it back and forth clockwise and counterclockwise to loosen it. It should loosen enough so you can remove it. Don't try to push it over, pry, tilt or pull on it, anything that will put pressure on the cpu/socket.