it it's a digital download, they must be confronted with this sort of thing by folks all the time. I would think you just give em a shout and tell them what happened. If they are reputable at all they will fix you up, after all... it's not as if the copy costs them anything. and it's not as if you having two copies would even hurt them. if it has to be registered a second copy would be useless, if not, you could be making copies all day and they couldn't do anything. I say honesty is the way to go, but ultimately it's up to you. If you lie, they might call your bluff and try to "help" you get past the reason it didn't work. then you have to explain that you deleted it anyway.
here's hoping all goes well and you get a new copy!