Based on your description, it sounds like you will need to perform a Master Reset on the MFD12 to bring it back to "life".
The steps for the MFD8 and the MFD12 are the same. I will list them below.
1. Disconnect any network devices.
2. Disconnect any NMEA0183 devices.
3. If the MFD8/12 is ON, power it OFF by pressing and holding the "Power" key for more
than 5 seconds.
4. Press and hold the [CANCEL] key. Do not let go of it.
5. Press and release the "Power" key to turn ON the MFD, but keep holding in that [CANCEL]
6. Eventually, you will see a cursor appear on the screen. Keep holding that [CANCEL] key
in! Continue holding in the [CANCEL] key until the Installation Wizard appears
(approximately 1 - 2 MINUTES).
Yes, I do mean MINUTES. This is not hard to do, but you may begin to wonder if you accidentally released it, just for a second. Or, you might wonder if you missed a step and are standing there, holding that button in for nothing. You may even think about letting go and starting over. Don't. Just wait.
7. Unlike the MFDBB, the MFD8/12 will not power off during this process. Finally, after it seems like you have been holding that [CANCEL] key in for too long, the Installation Wizard will appear, along with the language selection. (This confirms the Master Reset is complete)
Do not select the language. Simply power down the MFD.
If things went as they should, you can now power on the MFD.
At this point, it should start up and go to the Installation Wizard. Since you only indicate you have one MFD, you will set it as the Master and then click Save & Exit. Power it down.
You can now reconnect the interface cables and power it back on.
Go through the Installation Wizard to ensure all settings are correct.
If this process fails, you will want to get in touch with your local Authorized Furuno dealer.