Unauthorized music subscription charge after cancellation

First you need to determine what caused the charge. View your purchase history here:

View your purchase history for the App Store and other Apple media services - Apple Support

Also, review the subscriptions, to determine the start date and to cancel it:

View or cancel app subscriptions on iPhone - Apple Support

If you are the Organizer of a Family Plan, you may need to turn off Purchase Sharing or use Screen Time controls to prevent members from making purchases to your payment method.

If someone knows your Passcode/Password, those would need to be changed immediately. If your Apple Account has been compromised, then review this:

If you think your Apple Account has been compromised - Apple Support

You can try to request a refund. In most cases all purchases in the App Store are final, but Apple may issue a compassionate refund.

Request a refund for apps or content that you bought from Apple - Apple Support

The most important part is to figure out how it happened in the first place, so it does not happen again. If it was done on your device, it would require someone to know your Passcode to unlock it and then double click the side button with the amount showing on the screen, followed by entering the Passcode again. Extremely unlikely this would be done by accident.