I’ve had that happen to me quite recently. I fixed it by playing another song (it failed to play first time so I chose yet another song). Once it played that song, I went back to the original one I’d tried to play and it worked fine. I think it was a temporary blip. Maybe it will be for you. 

Failing that, turn the speaker off at the wall and close the app. Turn the speaker back on and re-open the app. 

To add to the useful advice provided, Sonos reps have acknowledged there are some issues with Apple Music on Sonos that may affect playback reliability. Just need to work around them for now as suggested above.