When you say "new" do you mean "unused" ? After a while is a SIM card isn't used the mobile companies remove them from the database as they can't have old/lost card data choking up their databases for ever.
If it says unregistered I suspect you would have to call your mobile operator. Or it may pay you to just buy a new SIM card. Some companies offer them free, and least some of the time. If you don't have their card, they can't make money from your calls.