Had this happen to a new boat last year after a few months of use. Ended up being one of the inner screws for the male plug connectors. Had to completely disassemble the plug to get to the screw – could see where it had worked loose, arced across and melted some plastic before completely losing contact. Has worked perfect since, not saying that is your issue, but one of those simple things that is not at the top of the list to check. Good luck!
this could short out the plug which would in turn trip the breaker(s). If it is one of the marinco plugs that do 12 or 24 volts you need to make sure the unused terminals inside the plug are tightened all the way down.
Either way, if the plug is bad, that wouldn’t result in no reading at the voltage gauge. There has to be a breaker connected to the batteries.