Hi Finn R.
I understand your Music app on your iPhone 6s is showing a blank screen and you are not able to access the music you synced from your iTunes Library to your iPhone. The first thing I would suggest is to look at your settings on the iPhone. Tap Settings > Music. Is "Show Apple Music" enabled (green)? If so, turn it off (white).
Next, sign out of the iTunes Store by tapping Settings > iTunes & App Store. Tap on your Apple ID, then tap Sign Out.
Restart your iPhone, and then sign back in to the iTunes Store.
If you see the "Splash Screen" (the girl with the headphones), at the very bottom of the screen tap "Go To My Music."
You should now see the songs you have synced to your iPhone from your iTunes Library on the Mac.
Finn, if the issue still persists, please reply back to this thread with more information, including any error messages you see, and we'll delve a little further into this for you.
The information above is a quick summary of the more detailed information in these resources for Apple Music and iTunes:
Use Apple Music in the Music app
![User uploaded file](https://support.apple.com/library/content/dam/edam/applecare/images/en_US/applemusic/ios9-iphone6s-apple-music-home-screen-update.jpg)
Sign in with a different Apple ID on your iPhone, iPad, or iPod touch
- Tap Settings > iTunes & App Store.
- Tap your Apple ID. If you see the option to Sign In, you can change your Apple ID now.
- Tap Sign Out.
![User uploaded file](https://support.apple.com/library/content/dam/edam/applecare/images/en_US/appleid/iphone6-ios9-settings-itunes-and-app-store-sign-out.jpg)
- Go to Settings > iTunes & App Store.
- Tap Sign In, then enter your Apple ID and password.
![User uploaded file](https://support.apple.com/library/content/dam/edam/applecare/images/en_US/appleid/iphone6-ios9-settings-itunes-and-app-store-sign-in.jpg)
Sync your iPhone, iPad, or iPod touch with iTunes using USB
Cheers!