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.

Spotify has a terrible network connection algorithm that will sit there and spin forever trying to open the app if it thinks you're on the network but you're really not.

Put your phone in airplane mode, then re-open Spotify, and magically, all of your offline playlists will play.