DIAGNOSIS HDMI-CEC (the feature that lets one remote control power and volume) is famously finicky. A software update on your TV or soundbar, or a temporary power glitch, can break the handshake.
FIX You need to reset the CEC communication. 1) Unplug both your TV and the Bose 600 from the wall outlet. 2) Wait a full two minutes. This drains any residual power and clears the memory. 3) Plug only the TV back in and turn it on. 4) Go into your TV's settings menu, usually under 'External Devices' or 'Sound', and find the CEC control setting (it might be called 'AnyNet+', 'Bravia Sync', 'SimpLink', etc.). Turn it OFF, save, and turn the TV off. 5) Now plug the Bose 600 back in. Turn everything on and go back to the same TV menu to turn CEC control back ON. This often forces a fresh re-negotiation.
IF THAT DIDN'T WORK The issue might be on the Bose side. Open the Bose Music app, select your soundbar, and go to 'Settings' > 'System' > 'Remote Control'. Ensure 'HDMI-CEC Control' is toggled on. You can try toggling it off and on here as well.
HEADS UP Some TV brands just have buggy CEC implementation. If this keeps happening, it might be more reliable to just use the Bose remote for power/volume and rely on your TV remote only for source selection. Frustrating, but sometimes more stable.