I just received an email response from Delta to my inquiry. They said my itinerary had timed out after I had placed it on hold and didn't pay for it (in miles) in the required amount of time.
I have never used a "hold" function at Delta in my life. The last time I used Delta award travel was a year ago to attend a family funeral. My need to travel was urgent. It all worked well.
Lesson learned: the next time I think I have purchased a ticket I should call Delta if I don't soon get a receipt with a ticket number. I'm lucky this was a leisure trip and that I didn't have nonrefundable ground arrangements.