Why are my recorded audiobooks missing from iTunes Music on PC?

i have recorded over a hundred audiobooks in itunes music, yet MANY have gone missing? Where are they?!!!

I have a pc. I run itunes on it. I expect Apple to keep all of my audiobook and music cd recordings safe and accessable. Yet I am missing many audiobooks, like the expanse series, that I recorded earlier.

Why??!!

[Re-Titled by Moderator]