DIAGNOSIS The remote's pairing with the TV has been lost, or the remote's batteries are failing.
FIX First, replace the batteries with fresh ones, even if the old ones seem okay. Then, re-pair the remote: Point it at the TV, press and hold the Input and Volume Down buttons together for 5 seconds until the remote light blinks twice.
IF THAT DIDN'T WORK Perform a system reset on the remote itself. Remove the batteries, press and hold every button on the remote for 30 seconds, reinsert batteries, and try the pairing steps again.
HEADS UP If the remote light doesn't blink at all with new batteries, the remote itself is likely dead and needs replacing—a common failure point for this model's stock remote.