I deleted one song from my iTunes

Where did it come from originally?

If it was purchased from the iTunes Store, you should be able to re-download it: click here-> Redownload music - Apple Support

If it was added from a CD or other source to iTunes and then synced to the iPhone, you would need to sync it the same way again. Click here-> Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

If I came from Apple Music subscription service, just find it again and add it to your Library: click here-> Add and download music from Apple Music - Apple Support