Thank you very much for your rather condescending and patronizing reply. I am thoroughly chastened vis-a-vis my obviously flawed back-up strategy.
I have a large back-up hard drive that is supposed to be regularly backing up the entire content of my hard drive, yet only some of my iTunes library -- a very small part of it, actually -- shows up in the backups, going all the way back more than a year. (Yes, I know. Check the back-up settings.) The folder indicates that it's the iTunes music library, but the content appears to be solely that of one of my "favorites" playlists. A substantial portion of my iTunes library has gone missing.
The curious thing is that all of my music (I have not checked all 700+ entries) appears to remain available on my iPad (have not checked my iPod yet). If this is, indeed, the case, it strikes me that one should be able to back up the iTunes library on my PC from the iPad. I don't use iTunes to play music all that often, but I'd like to find it still there whenever I want to do so.
Murphy aside, I feel my frustration is justified, albeit misdirected on this forum. Perhaps you and others feel it's inevitable that data just disappear or become mysteriously unavailable for no particular reason and are okay with blaming Murphy, God or Satan. I am not.
This sort of thing should be an extremely rare occurrence, yet it is not. I've seen dozens, if not hundreds, of posts on this particular failure of the Apple software and data management system, on which our devices -- none of which are inexpensive -- rely. Neither should one have to jump through a confounding series of hoops to recover data; most of us are not computer cognoscenti but mere users.
BTW, while I have had the occasional file corruption issue, no other files have ever inexplicably and suddenly (and randomly) disappeared from this PC, so I've never actually had to rely on my back-up drive at all. In fact, this marks the first time I've needed to recover data from a back-up drive in more than 25 years of computer ownership and use.
We users should be demanding more of Apple and its software; we should not all have to be computer and system gurus in order to deal with the deficiencies of Apple's system. If I had to guess, my tunes went missing as a result of one of the proliferation of operating system and iTunes software "upgrades."