Check the account being used there by tapping on the user icon at the top.
With that said, your account should not be relevant to content you synced directly from a computer using a USB cable.
What exactly did you do? Did you follow the steps in the link provided previously?
Was the entire music library selected or did you only sync specific playlists or albums?
In the Music app on your iPhone are you tapping on the library tab? Or are you trying to access the "Home" or "Listen Now" tabs?
You need to explain exactly what you did, and what you are doing on the iPhone now before we can offer any further advice.