[Geoff Bacon]

Songs is a list of all your songs so if you delete it from there it will be gone (as will all references to that song in collections or setlists).

You cannot delete songs from a setlist or a collection; it just removes the reference to the song (which is still listed in Songs).

All is not lost if you have manually used MS to take a backup.
I believe MS also takes automatic backups but I don't know how long these exist before they are overwritten or deleted

Geoff
PS I recommend that you always make a manual backup (Companion or MS) before making major changes to how the files are managed