Thank you,
@Brogan. My problem in this case was that it was only a two-person conversation, and while I remembered the subject, I'd forgotten who started it with me.
As it so happens, I took my best guess, asked in a new PM, and it turns out I did remember after all, so in this case I was able to continue, but in the event I hadn't remembered correctly, I don't see a way of looking at all my deleted PM's and restoring one or more of them.
I believe the work-around here is for me to have a second user account because some of what I get is perfunctory in nature, e.g., someone who has completed a specific course of ours asking to be granted access to a forum section for just those people. Those people all get an email saying, "If you're interested, please PM Steve Freides to ask for admittance." It's not something we can fully automate because event registration includes people who've yet to join our forum, or who joined using a different email address than the one with which they registered for the course. It was a pile of these perfunctory PM's that I was deleting when I mistakenly deleted a PM I had promised to follow up on.
Further thoughts and suggestions are welcomed, and thanks in advance, as always.
-S-