If you have previously established a connection between the Sensor and the Manager and the connection fails, try the following:
Check network connectivity.
View the system status on both the Manager and the Sensor.
Check to ensure the Management port on the Sensor is configured with the proper speed and duplex mode as described in Management port configuration.
Has the time been reset on the Manager server? The connection between the Sensor and Manager server is secure, and this secure communication is time-sensitive; so the time on the devices should remain synchronized. You must set the time on the Manager server before you install the Manager software and never change the time on that machine. If the time changes on the Manager server, the Manager will lose its connectivity with the Sensor and the Update Server. A time change could ultimately cause serious database errors.
For more information, see the KnowledgeBase article KB55587.