400+GB music collection disappeared while listening to music/ Catalina just randomly crashed and gone

Okay, first, I do not subscribe to Apple Music service. My music collection was gathered over time from CDs and various digital sources. I have access to the entire library in my Time Machine which is connected to a 2TB external hard drive. It is there. I have been slowly adding it back into the Music app, about 4GB at a time. This all very time-consuming, but when I had attempted to reload the entire library at once, about halfway through it stopped the loading with a message that my storage capacity was short by nearly 200GB. Ergo, I did the library search in the music app, but it said "not found". Same with Spotlight. It's only in my backup. My inquiry has more to do with why I am even having to do this at all? I never had these problems with iTunes for years and now this new Music app has everything all screwed up. I'm just thankful I backed the system up.

What I really was interested in was how to access my music collection without running it through this new Music app. Wish I just had iTunes back. I'm not very happy about my highly organized collection of music that took years to put together getting trashed by Apple with their stupid "upgrade". I have an adage that I've used for years: "If it ain't broke, don't break it!"

To answer your questions:

1. No error messages or alerts before shutdown occurred. After logging back in upon startup, music library had been reduced to 24GB from over 400GB. The particular music that was playing when this happened was missing.
2. I am only aware of music being missing, but that was easily my largest file.
3. The primary change was when I updated to macOS 10.15.6 (Catalina).
4. One update left me with a login screen with the top third missing, but login box was still accessible. After login, screen returned to normal, but partial screen was always there when waking up. That went away with an unannounced update. There have been multiple "surprise" updates with no warning with Catalina, and each time Music would take an extremely long time to complete loading, but would load eventually. I know it's a very large file, but this last time the system restarted unexpectedly, the file was not loaded at all, except for the most recent additions, of which only two albums were from earlier this year. I've been reluctant to continue adding to my collection since these issues with Music on Catalina.

So, I basically answered most of my own questions, while everyone else answered nothing at all. I would still like to know why this problem I had happened at all in the first place. I took care of the problem that was caused by Apple's "upgrade" to Catalina by painstakingly reinstalling everything slowly and meticulously using my external backup through Time Machine. At least that works properly. I still have lost a great deal of album art which I have to correct as I notice it's missing. I have other questions, but at this point I don't see any evidence of anyone on this forum has much to offer. Thanks to everyone!

Another observation:

All the music that did load in the music app has the little cloud symbol by it, indicating it is only in iCloud.