Years ago at my last confession, I was told to do a certain penance for a mortal sin. I did not carry it out. Now I cannot because the place of business is closed and I have no way to locate the person. What should I do?
If you have not already done so, go to confession and confess having neglected the penance you were given. If you have done all that you reasonably can do to locate the person, you need not worry further on that score. If the penance was of a type that it can be done for another person or organization, if you wish you can do that in lieu of doing it for the person or organization that you harmed. In other words, for the sake of example let us say that this involves monetary restitution. If you cannot find the person or business to whom it was owed, you can give the restitution to a charity.