From the Model Mayhem FAQ:
My card declined, but my subscription is still active. What next?
First, check your decline reason. That will tell you a lot about what you need to do to correct the error.
Once you know what to do to fix the error, take steps to correct it. If you need to contact your bank, do so. If you need to update your card, follow the steps to update your credit card or add a new one from your Wallet.
The system will automatically attempt your card information within 7 days of your billing date, so if you update your information there is nothing left to do. However, if the decline reason is not corrected your account will downgrade automatically.