The no decodable tracks for a song that was there last year, usually just means that the itunes library removed the local copy of a song to make room on the device. You can usually fix that by opening the music app on the device and tapping the cloud icon by the song to redownload.
The problem with it not opening the itunes library is usually something hung up in the iOs music app. That entire picker screen is actually run in the music app process. I would try either killing the music app or restarting the device.