The media are still there and apparently all accessible; only the playlists vanished. And it turns out that iTunes Media is still there as C:\Users\<User>\Music\iTunes\iTunes Media; thanks for prompting me to check more closely. But . . .
a) In Windows Explorer, C:\Users\<User>\Music\iTunes\iTunes Media contains only an empty folder called Automatically Add to iTunes and a folder for iPod Games. No Library or Previous Libraries.
b) Nonetheless, the Windows Explorer search function locates files entitled C:\Users\<User>\Music\iTunes\Library and C:\Users\<User>\Music\iTunes\Previous iTunes Libraries\iTunes Library <date>.
c) Meantime, all my media have been relocated from the iTunes Media folder into the parent folder C:\Users\<User>\Music\iTunes\ (which explains why I didn't spot the iTunes Media folder among them).
The security permissions appear to be set correctly, per your post. I also tried using edit, preferences, advanced to change the location of the iTunes media folder to just C:\Users\marie_000\Music\iTunes. No libraries appeared, so I set it back. Any other thoughts?