This has happened twice in the last week; whilst listening to Spotify my phone ran out of battery. When I charged it back up and reopened Spotify, I had to log back in and re-download all my music that I had already on my device before my phone had died. This hasn't happened in the previous 4 years that I've been using Spotify.
First, I advise that you check the download status. Successfully downloaded tracks (on mobile) and playlists (on desktop) should have the green downloaded icon (
) next to them.
If they don't, or if the arrow is grey, we’d recommend checking the 'Music is not downloading' steps here, and redownloading.
If they do, try switching on Offline mode within the app’s Settings, or switching off your device's internet access. This ensures that the app doesn’t try to connect to any available networks which can affect playback.
If this doesn’t help, try reinstalling the app and re-sync all of your offline music. I know this maybe a tedious work to do, but it should keep you going.