01 2500 TD not start - code P ECU?

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,
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