Diagnosis
The universal control settings got out of sync. The TV needs to relearn the infrared codes for your specific device.
How to Fix It
Press the Settings button on the remote. Go to All Settings > Connection > Device Connection Settings > Universal Control Settings. Select 'Set Top Box' (or your device type). Choose 'Auto Set' and follow the on-screen prompts—it will test power and volume commands until it finds a match. Select 'Yes' when it works.
If That Didn't Work
The cable box might use a non-standard IR code. In the same Universal Control Settings menu, choose 'Manual Set' and try selecting different brand codes for your provider (e.g., try Xfinity, then Comcast, then Arris).
Heads Up
The magic remote uses a combination of Bluetooth (for pointer) and IR (for device control). Make sure you're pointing the remote at the cable box when testing volume/power, as the IR blaster is at the remote's tip.