When an account exceeds the number of remaining credits purchased, the account will not let you send out any further emails unless you have the Allow Overages extra enabled. With Allow Overages, the system will charge you for sending over your maximum limit. These overages are billed to your account at the end of your month's subscription, or when you upgrade or cancel an account.
If you are sending a lot of extra emails, consider upgrading your plan to a larger package. You can upgrade your plan through the My Account Menu by selecting Subscriptions.
You may also consider purchasing pay-as-you-go credits, which can be used with your campaign and do not expire as regular credits do from a monthly subscription.
If you are sending to the same email list frequently, and this list is 30,000 contacts or less in size, another option is to purchase a contact-based subscription. A contact-based plan lets you send to your list of contacts an unlimited number of times each month, provided you don't exceed the number of contacts purchased in the lists you allocate to be part of your contact-based plan.