When a person prays and is not answered, he should not give up. One of the reasons why we pray is not to change G-d’s mind, but to change ourselves. The Chovos HaLevavos states the reason we pray is to realize everything comes from G-d. When we realize everything is dependant on G-d, it changes us to a different person. Therefore, since we are now a new person, a negative decree issued against us is no longer valid because we are no longer the same person.