Diagnosis
The remote's pairing with the TV has been lost, or the remote batteries are critically low (even if they seem fine).
How to Fix It
Re-pair the remote. This fixes 95% of cases.
1. Point the remote at the TV and press and hold the 'INPUT' and 'VOLUME DOWN' buttons together for 5 seconds. The remote LED should blink.
2. Wait 30 seconds. The TV should respond. Test the power button.
3. If no response, replace the batteries with fresh ones (AAA) and repeat step 1.
If That Didn't Work
Perform a power cycle on the TV (unplug, hold physical power button for 30 sec, replug). Then try the pairing steps again with fresh batteries.
Heads Up
Avoid using rechargeable batteries in this remote; their voltage profile is different and can cause premature 'low battery' behavior even when charged.