I read two symptoms.
1) that when you play a file you get a #2.
2) the later the SDcard won’t initialize.
The first symptom would indicate that you may have gotten past the SDcard initialize, when running the demo. Assuming your were attempting to run 
MP3Shield_Library_Demo.ino
Note that the native SdFat examples are not aware of the VS1053. Hence the native SdFat examples do not disable the VS1053’s chipselects. Rather they leave them floating. Which can cause the native examples from properly initializing, if the VS1053 floating chipselects float in the wrong way. I always tweak the SdFat examples so that D8 (VS1053-Reset) is LOW, so that the VS1053 won’t interfere with testing the SdCard.
It is also worth testing different SdCards of different make and model.
If #1 is true and the demo did initialize the SdCard and you got a #2 error while attempting to play a file. It is likely not named properly. I originally used the SdCard Example that would dump the directory. However, you can try to use the FilePlayer.ino it will list the directory and allow you enter an index of the file you want to play.
