This was the best and easiest solution when my iPhone Music app was stuck on a white screen:
1. Double click on the Home button
2. Swipe all the app screens upwards one by one until only the home screen is left
3. Click on the Home button
4. Press and hold the Sleep / Wake button until the screen shows something like "Swipe to turn off"
5. After 30 seconds or so, press and hold the Sleep / Wake button to restart the iPhone."
Source: Music App show white Screen | Official Apple Support Communities