Upon the reboot, the "/" filesystem was mounted "read-only". I won't go into all the rabbit holes I've been going down for hours.
I'll just say that once I tried running "mount -o remount,rw /", I received an error along the lines of "unknown option 'nodiratime' ", (Sorry can't remember it exactly) an option I've had in my /etc/fstab file for months!Sure enough, once I removed that option from the "/" line in fstab, I was once again mounting "/" in read-write mode.
Posting this here 1) to see if anyone else has seen this recently. and 2) in case someone else finds themselves in the same pickle they'll know how to fix it.
Finally, in researching this option, I found a post indicating the "noatime" option implies "nodiratime".
So it appears, at least from my perspective, that what used to be a valid option in fstab is no longer valid, at least not in combination with "noatime".
Hope this helps someone.