DIAGNOSIS: The TV's Bravia Sync (HDMI-CEC) feature has likely lost its programming for your soundbar. A software update, power glitch, or changing inputs can sometimes cause it to forget the linked device.
FIX: You need to re-register the device. 1) Press the Quick Settings button on your Sony remote. 2) Go to Settings > Remotes & Accessories > Bravia Sync device list. 3) Find your soundbar in the list. Select it and choose Re-register device. Follow the on-screen prompts, which usually involve turning the soundbar off and on again. Once it's found, make sure System Audio Control is turned ON for that device.
IF THAT DIDN'T WORK: Go nuclear and reset the entire Bravia Sync system. In the same Bravia Sync settings menu, select Bravia Sync control and toggle it OFF. Wait 10 seconds, then toggle it back ON. This forces the TV to rediscover all connected devices and re-establish control.
HEADS UP: Some soundbars have their own CEC setting that needs to be enabled. Check your soundbar's manual (I know, I said I wouldn't say that, but this is external) for an option called 'HDMI Control', 'CEC', or 'System Control' and make sure it's on. Both ends of the wire need to be talking.