Thank you, Vilas_S, for your reply!
All the contacts are stored in iCloud but restoring them neither restored this most recent inexplicably vanished contact nor any of the countless previous contacts that similarly inexplicably vanished while they were being modified. In case this might help, the "Last Import" Smart Group is empty.
This iMac is THE MOST problematic computer I've ever used. Since I first turned it on, a multitude of ever-changing problems has beset even the most simple of tasks, the startup volume dramatically changes from startup to startup (even though the Alert and Output volume settings remain the same), it instantly shuts-down without warning but doesn't offer to send Apple a report, sometimes it refuses to start even though TWO new power supplies have been installed, Mail is interminably slow, Calendar additions and changes don't always save or populate to future dates . . . the list of senseless, aggravating, time consuming, data-destroying problems is endless. Immediately after the warranty ended, the Apple Store said that this then-still-relatively-new iMac wasn't worth repairing and they "should have" replaced it yet they continue charging me for incessant repairs. Apple Care "was" in the process of attempting to help but after the technician again asked me to change settings, until he could continue helping (leaving my computer in a vulnerable state), he never contacted me again; when I asked Apple Care what happened to "Tony," they said they closed my case, having never notified me, and I would need to begin again.
I'm a home-bound, 100% disabled, American Veteran completely dependent on technology yet this "Faulty-from-the-Factory-iMac" has been profoundly more bane than benefit but being unemployable for three decades forces me to expend precious time, consideration, and effort addressing all it's too many recurrent problems. ARGH!