My older Onan MDKD stop/start solenoid has 2 "holds". One pulls the plunger across to allow fuel to flow to start, and after it starts the other one holds the plunger across all the time, so fuel flows. Your second "hold" could have failed. Can you look at it to see what it is does during the start process. You might be able to hold it across manually and see if it still runs.If it has failed the only solution is a new one, and for that, esp if you have the part number, check ebay as well as your Cummins guy.
But of course it can be something else, as others suggest.