The wireless controllers were not reporting an outage or that any APs were down. However, clients were unable to connect to our secure networks (Marauder Guest was operational), which indicated a failure of communication between the WLC and ISE. ISE logs did not show any client activity, so the problem was likely with the controller. I rebooted the controller stack, and that remedied the problem. The stack is back up, and all SSIDs are functioning correctly.