Also, what people may have not mentioned is that every region card doesn't work for other regions except it's specific. So if you traded someone for that card, and it's not your region, then it won't be working for you. Sorry to say.
If that's not the case, however, you can give Sony a call if you're sure that the code should be working. From there, they will take measures to make sure that you're telling the truth and that the code should be working and if you are proven to be correct, they will surely transfer the funds over to your PlayStation Network account.