Suddenly he is fired, or the business fails and he is out, or he may be retired too early. His world has come apart. Another person commits a crime and suffers public disgrace. Doors are shut to him which used to be open. Prison means mostly that the world is shut out and the prisoner is shut in.
Life changes completely - old relationships are ended or strained until they are no longer the same.
Life's major commitment, whatever it may have been, can no longer be followed. The future looks bleak. The prisoner's world falls apart. A deadly illness limits another person's life span. Or a crippling disease or accident terminates his major commitment, and his relationships are ended or drastically changed.