This sounds very familiar as I just went through this same routine after defusing my Fusion drive. iTunes is looking for the iTunes Library FILE that has the pointers to the actual tune files and is normally found in ~/Music/iTunes (where ~/ indicates your user home folder). According to your settings when you loaded the tunes the actual tine files may be located in that same ~/Music/iTunes folder or they may be scattered in music files all over your hard drive and even on other attached drives, or even a combination of both. Having everything in a single iTunes folder is preferred and far easier to manage. In fact there is an iTunes preference setting to copy the scattered files into the single folder.

Try this.
  1. In Finder locate you itunes library file (it has the edxtension .itl).
  2. Hold down the Option key and launch iTunes. You will be prompted to
    1. Create a new iTunes Library or
    2. Choose the iTunes Library.
  3. Choose b and then the folder containing the iTunes Library you found.
  4. If your iTunes Library is still messed up you can try creating a NEW iTunes Library and pointing the folder where the bulk of your tunes are located
  5. If you have scattered music files you may have to import them again. You won't need to go back to the original media just use the files that are somewhere on your system.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein