I had the same problem just Sunday in a 340 Cuda that had added Mopar electronic ignition.
It therefore had the 4 prong ballast resistor.
It had never done this before, but crank and crank and release the key it would start, otherwise not.
Checked some wiring leading to the ballast resistor and noticed one of the top terminal connectors was loose.
Crimped it tighter and 'voila' started and ran like it always did.
Hopefully it's something simple like this was.