2008 Ram 2500 Transmission problem? Vapor lock problem?

Hi, I was driving through a campground tonight and my truck (2008 dodge cummins diesel 2500) just went outta gear. No warming, no nothing. It did go in reverse so I was able to park it. I shut it off, restarted it, and it went into D.
Any ideas as to what the problem may be?
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Answers
Vapor lock is a myth left over from the 1970s. Does not apply here, or to any diesel engines.