This has happened before. My iTunes library is made up of downloaded albums as well as CD's I have purchased and imported. Every so often, one of those CD's will disappear from the library. Any clues ?
For most people it's "would that there were one". Your advice in every post is sage.
After some scares with RAID crashes in the past, I now back everything up using software that does an initial complete backup followed by daily backups of anything that has changed. Had a little scare yesterday when I thought that I may have lost my portable hard drive at a hotel in Sacramento. (I keep all of my music files and MediaMonkey on it using a portable installation so that I can listen to anything I want via my laptop when traveling.) I worried a lot less knowing that a full and up to date backup was waiting at home if needed.
The newer, cheaper high-capacity hard drives (particularly Seagate) seem much less reliable that the ones of a decade ago. I have twenty year old drives that still work fine, but have had three new Seagates fail in the past two years, none older than three months. Back up everything you care about! (And don't trust RAID, IMO.)