If your synced music has been deleted from the Music app on your iPhone, there are a few things you can try to recover it:
1. Check if your music is still in iTunes or Apple Music on your computer: If you synced your music from your computer, check if your music is still in iTunes or Apple Music. If it is, you can try syncing your music again to your iPhone.
2. Check if your music is still in iCloud: If you have iCloud Music Library enabled, check if your music is still in iCloud. Open the Music app on your iPhone and go to Settings > Music > iCloud Music Library. If it is enabled, turn it off and then turn it back on. This should refresh your music library and restore any missing music.
3. Restore your iPhone from a backup: If you have a recent backup of your iPhone, you can try restoring your iPhone from that backup. This should restore your Music app and all your synced music to its previous state.
4. Contact Apple Support: If none of the above solutions work, you can contact Apple Support for further assistance. They may be able to help you recover your missing music or provide other solutions to the problem.