If the sin was venial, it did not have to be confessed and there was no sin in not mentioning it. If it was mortal, the confirmation was valid but the person is not open to the graces of confirmation until he goes to confession for the sin. What such a person should do is to go to confession, confess the original sin, the sin of hiding a mortal sin in confession, and the sin of deliberately receiving confirmation when one knew that he was not in a state of grace. Once that is done, the person will not only receive the grace of absolution, but he will also now be capable of receiving the graces of the sacrament of confirmation.
