You can contact Citibank through several channels, but Citi recommends logging in to Citi Online or using the Citi Mobile App. You can also connect via:
Phone: Call the number on the back of your card or call 1-800-950-5114 (General Customer Support & Services).
Appointment: Book an appointment to visit your closest Citi location.
Mail: If for any reason written mail would be your preferred communication, this is Citi's general correspondence mailbox: Citibank Customer Service, P.O. Box 6500 Sioux Falls, SD 57117