You probably have a registered domain with RespectMyPrivacy service. That is the only service intentionally allowed to overdraft your account. We could very easily cancel or disable it as soon as you run out of funds, but the consequences of doing so are permanent -- once your information becomes public, reactivating privacy later won't cause people to unsee it or remove it from websites that troll whois info looking for and publishing these kinds of changes.
So, because we care about your privacy and we know that oversights happen, we choose to give you every opportunity to correct the problem before we have to expose your personal details. Our policy is to cancel your RespectMyPrivacy service after 30 days of nonpayment. However, during that time, charges will continue to accrue, leaving you with a negative balance. That negative balance directly reflects that we would rather lose money than let a brief lapse compromise your privacy. Also, please keep in mind that privacy service is still needed for about 75 days after the domain expires.
The best way to resolve this is to add funds. If you don't want to do that, you can also remove RespectMyPrivacy to prevent further charges from accumulating, but the negative balance will hang around (and generate reminder emails) until you resolve it or close your membership.
The best way to avoid this is to prepay your RespectMyPrivacy service. You'll even get a 10% prepayment discount for doing so. This option is so good that it's now baked right into the service, but it's not retroactive so if you have an existing domain without it, the change won't take effect for existing domains until you renew them or manually prepay. If you are reading this because non-prepaid privacy resulted in an overdrawn account, that's an indicator that manually prepaying is probably right for you.
The best way to check this is to go to the Account Information panel for the overdrawn account and look at the "Estimated Recurring Activity" box. It will contain a "RespectMyPrivacy Service" line showing how many domains you have with privacy service, and how many are prepaid.
The rest of our service is fully paid in advance; charges (and services) automatically suspend when your account runs out of funds. In some rare cases, you can wind up with a negative balance of a penny or two if your usage is just right at the moment your account is depleted. If that happens and you don't want to add additional funds, don't worry, when you cancel our system will scratch out those types of small negative balances.
Naturally, issuing chargebacks can also result in a significant negative balance. Any chargeback-related negative balances must be resolved before you can cancel or access any services paid for by the charged-back funds. (For example, chargebacks are one of the few justifications ICANN allows for a registrar to suspend or seize a domain name.)