This issue often happens due to a brief interruption in an internet connection. Please try quitting and restarting the app while making sure you have a strong internet connection, as this may resolve the issue.
If not, you can also try deleting the session from 'Manage Storage', then playing it again.
You can delete session and scene files from Settings in the app. In Settings, you should see a 'Manage Storage' option. Under that, you should see delete 'all meditations', 'all music', 'all sleep stories', and 'all scenes'. Tap the option you would like to remove from your downloaded files.
Once sessions or scenes are deleted, they can be re-downloaded at any time.