T-Mobile has admitted that it recently suffered a data breach (via TmoNews). The carrier says that it recently discovered and shut down unauthorized access to prepaid accounts. The issue has been reported to the authorities.
Customers' financial info, social security numbers, and passwords were not compromised in this data breach. However, T-Mobile does say that prepaid account info like name and billing address was accessed. Also accessed in this data breach are customers' phone numbers, account numbers, rate plans, and additional features like an international calling add-on.
T-Mo is in the process of notifying all customers that've been affected by this security issue. If you didn't get a notification, it likely means that your account wasn't compromised, but it is possible that T-Mobile isn't able to notify you because your contact info isn't up-to-date. If you think that that's the case with your account, you can contact T-Mo by calling 611 from your cellphone or 1-800-TMOBILE from any phone.
Data breaches like this one can be frustrating because it means that a malicious party may have accessed your info even though you didn't do anything wrong. While it's good that no financial info or passwords were accessed in this incident, customer data like names and billing addresses were compromised, and that's sensitive info as well. If T-Mobile has told you that your prepaid account was accessed, you should update your PIN as soon as possible.