If the pump is running constantly then it kinda boils down to a few things. Its either sucking water and dumping it, or its sucking air and wont prime. First, as previously mentioned in the above posts, do you see any water anywhere? No, is this a new problem with no changes to the supply line from the tank? You did it just like every other time? No, well then it is probably not some sort of closed valve. If it aint sucking water, then it is the sucking air part. Loose coupling? Maybe but not likely. Does your pump have a small strainer with screen right in front of the pump? Possible clog? Finally, its possible the diaphragm pump could have failed.
Also, you said you filled the fresh water HOLDING TANK. I associate that with black. Think your "belt slipped" there. LOL. Mine does all the time. And of curse I know what you meant.
Good luck
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