First thing you should Contact the club. However, they will normally only issue re-prints an hour or less before kick-off.
But If you have ordered your tickets within 2-3 days of a game then the Ticket Office will not post them out. You will have to collect them from the ticket office window on the day of the game (the Ticket Office is located at the Kop end of the ground)
If you have ordered them more than 3-4 days before the game then you will have to turn up at the Ticket Office on the day of the game with appropriate ID (fancard, the card used to buy the tickets, possibly drivers license or similar if you do not have a fancard) and they will issue a re-print/duplicate. Note that the Ticket Office tend only to do this within approximately 90 minutes of kick-off.