Not sure why they were, but I tried 6 different offline playlists and each of them said "can't play the current song", then cited internet detection... which hadn't been an issue.
Now (10:04a) all of my playlists are working again.
Now (10:04a) all of my playlists are working again.
Put your phone in airplane mode, then re-open Spotify, and magically, all of your offline playlists will play.