iTunes issues - NOT apple music related

Okay, I am a dance teacher, who occasionally buys new music via iTunes, 96% of what I have has been added to my iTunes over many years from my CD collection which is currently in storage.

I keep my iTunes on an external hard drive in an attempt to not slow dowm my late 2015 iMac that I also use for FCPX, all apps, os, etc are up to date.

I was creating a new playlist in iTunes to then transfer via sync with to my iphone 11Pro. I played a song, then went searching around for a few more, decided I wanted that one from 10-15 min ago and low and behold iTunes couldn't find it.... I then went looking in the media folder on the external harddrive and it is as if that album NEVER EXISTED.....

WTF? 🤯 🤬

I would appreciate any advice. This has been happening a bit that iTunes can't find such and such, but I haven't been following it up until now... I just hope I can find the CD in storage, as the album ain't in the iTunes store!

Cheers

Shannon