Based on your information, you have what we call the BBFF1 network sounder. If there is no bottom, speed or temp being displayed on the sounder page, please check to see if the BBFF1 has power and the "ST-BY" light (next to the network connection) is blinking after 3 minutes. If it is, the module is on and is communicating with the hub/switch or with the NN2. Check to ensure the menu settings didn't get changed, under Menu-System Configuration-NAV Option-NAV Source Settings, ensure Depth, Speed & Temp sources are set to ETR. If you don't have power or blinking ST-BY light, then troubleshoot power to the BBFF1.
If everything is good (from the above checks), verify communication to NN2 by running the network sounder test by pressing Menu-System Configuration-System Setup-Test & Clear-Memory I/O Test-Network Sounder Test; if a black screen with a program number is displayed, then there is good communication. If it quickly flashes a black screen and goes back to the Memory I/O Test page, then it's not communicating. Most likely cause would be an Ethernet cable or HUB/switch problem. If all tests show communication and operation, then the transducer has gone bad and needs replacement.