Ipod (5th gen) music library gone

I recently revived an old 5th gen Ipod, which includes a lot of MP3 file audiobooks and some music. All of these were in the "music" folder. All was well. I plugged it into my iMac; Apple Music shows the iPod contents, and I was able to drag and drop a few more files/"albums" from my computer into the iPod. However, upon unplugging and trying to use the iPod itself, the "music" option at the top of the menu is gone. The files are all still there; using "Shuffle songs" accesses them (but hardly helpful when you want an audiobook in chapter order). What can I do?