I plugged a new iPad into my PC and lost old CD ripped songs from my iTunes library

My iTunes library on my PC was full of music that I ripped from old CDs, which is also on my iPod touch. I recently got an iPad pro mini to replace the iPod touch (I don't like having all that music stored on my phone). I set up the new iPad and plugged it into my PC to transfer the full music library from my PC to my iPad, and much to my chagrin the opposite occurred. Everything that I'd previously uploaded into my iTunes library disappeared and left me with only the music I've purchased through iTunes from the cloud. I've checked everywhere for this in iTunes and on my PC and it's nowhere to be found...I've made sure that I'm not just looking under that device

First, I'd like to know why this happened so I don't lose all of the music off of my iPod touch if I plug that into my PC now.

Second, I'm wanting to know how to get all of that old music onto my iPad now. I've found an old external hard drive that has all (or at least most) of it from when I got a new laptop and transferred it. I'm seeing lots of similar questions, but different answers that seem to address only music purchased through iTunes or Apple Music, or stating to do exactly what I actually did which resulted in losing all of that old music from my PC iTunes library. I want to have access to all of my music on my iPad, including those songs ripped from CDs.

Lastly, I'd love to know other safe and successful ways that people have used to protect from losing their music libraries.

Thank you!