2001 Cummins 2500 6-speed code P ECU?

From : charles

Q: i stopped for fuel on the way home. i did not remove the keys and just turned it off one notch. after fueling it would not start. where the mileage is normally displayed it showed p ecu. when i say it would not start turning the key to the start position did not do anything no click no crank no nothing. voltage was normal. finally i removed the keys and put them back. then it started and ran normally. it was not driven hard before i stopped. what does that code mean truck is 2001 dodge 2500 w/cummins td and 6-speed manual. thanks for any advice charles .