I have had a knee locking problem for about 16 years. Can any one help? Usually I can 'click' it out.?

I've had this problem for a long time, usually I take a deep breath and click the knee back in. It makes an audible clunk and it's fine but It's been locked now for about 2 days and I can't click it out. It is becoming very painful. I am 55 years old. I have never had any injury to the knee but have been told it is related to the cartilage. I do not want to have surgery.