1. First of all sync your iPhone with the computer.
2. If you skip the first step, then “offload” the Music app from Settings>General>iPhone Storage>Music [Warning: I haven’t used/checked this before. My 6th sense feels so.]
3. Sign Out iCloud/iTunes/whatever account from your iPhone by going to Settings>iTunes And App Store. Click on your ID to Sign Out.
4. Sign in again with the same ID.
5. Check Music app. If it works fine then it’s OK. If not, follow the following steps.
6. After Signing out from apple ID, don’t sign in again before restarting the phone. Restart you phone.
7. Go to Settings>iTunes & App Store>Sign in.
8. Open Music app. Enjoy!
PS: You might lose your already downloaded music or playlist.