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From the Model Mayhem FAQ:

My credit card just got declined, and my subscription is still active. What do I do?
The decline notification you received will likely have given you a reason for the error. To correct this, simply go to the VIP link on your My Stuff page, where you'll be taken to your 'wallet' of credit cards. You can either edit your credit card information here or add a new credit card to use for your membership. If no reason was given for the decline it is probably easiest to simply go to your 'wallet' and resubmit your credit card information or add a new credit card.