From the Model Mayhem FAQ:
My card declined, and my subscription was downgraded. What next?
Once your account has downgraded, the only way to resubscribe is to go to www.modelmayhem.com/upgrade and re-choose your preferred subscription method.
If your card was declined for an error, and you attempt to use that same card again, you may not have your payment process, and will need to correct the problem before you can resubscribe.