My phones service is not working I think someone hacked my account

Which account: Your Apple ID or your cellular provider service account ... or both?

For your Apple ID: If you think your Apple ID has been compromised - Apple Support

For your cellular account, you will need to contact your cellular provider.

You mentioned that your phone is locked. Which kind of locked: Activation Locked, IMEI Locked, Carrier Locked, Company Locked, or locked out because of the Passcode?

If you mean your cellular network account contact your carrier. If you mean your Apple ID account see→If you think your Apple ID has been compromised - Apple Support