Erased music by mistake

Hello! I've definitely been in your position before. Do you have iCloud syncing enabled for Music? Here's how to check if you do:

1. Open Settings
2. Tap [Your name] > iCloud > Show All

If the switch next to 'Music' is off/grey, you may be able to restore your music library from an earlier backup.

iPhone automatically backs up to iCloud if:

• It is signed into your Apple ID
• It's able to access a network
• There's enough iCloud storage space

Here's how to restore iPhone from an earlier iCloud backup:

1. Erase all data from your iPhone: How to factory reset your iPhone, iPad, or iPod touch - Apple Support
2. Follow the onscreen setup steps until you reach the Apps & Data screen, then tap Restore from iCloud Backup. 
3. Sign in to iCloud with your Apple ID.
4. Choose a backup. You should be able to see the date of each backup. Choose the one from before you erased your music.

(Restore your iPhone, iPad, or iPod touch from a backup)

I hope this helps!

It sounds like the song has become hidden. To unhide a song open the iTunes Store, scroll up to the bottom and tap on your Apple ID to view it, then scroll up to Hidden Purchases > Music and unhide purchases.