What do you mean by "unblock"?
If you can't remember your lock screen passcode, then restore it from backup. If there is no backup available, put it in recovery mode and restore it as new.
If you mean unlock it for use on a different carrier, contact the carrier it is locked to. They are the only ones who can authorize that.
If you mean the phone has been blacklisted, then pay your bill or buy a phone that's not stolen.