Local files disappeared?

[Ponchissimo]

Hello everyone! I've been a premium user for almost 2 years now and have always felt satisfied with it. A few months ago, I downloaded the "regional at best" album from 21 pilots on my desktop and synced them as well on my iphone using the tutorial on the website. Today I was trying to download a new song and repeated the same steps i did before. Sadly, that song kept on not syncing on my phone and when i tried to modify something in the settings (turned the "show unavailable songs in the playlist" switch from off to on) I ended up with all the old songs from my local files disappearing from my iphone SE. What can I do to have them back on it and add new ones?

thank u very much!

edit: the playlist in which the songs were, still shows "12 songs" on my iphone even tough no playlist covers appears nor song shows up when i click on it.