If you find your account suddenly unable to send mail within the first week of the month, your account could be in a billing frozen state. This occurs when the card on file for your account is declined when we attempted to process payment on the first of the month.
To reactivate your account please update your credit card on file by logging into your SendGrid dashboard and going to Settings > Plan and Billing Details > Change Billing Info. You can then change or re-enter the card information and then click Save. Once you update the card on file, the payment will process automatically and the account will be unfrozen.