Just a wee report of what happened to my NMEA2000 network, incase the same thing happens to somebody else. The other day whilst crossing from Knoydart to Mallaig in heavy chop my Lowrance 332C reported that there was no communication with the GPS unit, comms to the fuel flow sensor (an EP-10) had also failed. I shut the engine off and power cycled the all the electrics using the battery isolator, no luck. After a bit of investigating today, I noticed that the fuse which powers the NMEA2000 bus had blown. I replaced the fuse and it blew after a minute... Turns out that there was continuty on three contacts in one of the T-Pieces on the bus (there isn't on any others, I'd removed att of the terminators) - replaced it with a spare and put another fuse in and all is well! I can only think that salt water ingress must be the cause, but they are supposed to be waterproof and they were inside the console (damp, but not wet)...
Hope that helps someone!