Try this: Open the battery compartment on the back of the remote and remove both batteries. Then press and hold any button on the remote for 20 Seconds, release the button and reinsert the batteries.
If this does not work try re-pairing the remote: Press the RETURN & GUIDE buttons simultaneously for 3 seconds. Then release the buttons but keep the remote in front of the TV. Watch lower left of screen for pairing info. If the pairing is successful, you can now - hopefully - use it!
Hope this helps.