DIAGNOSIS The remote's IR/Bluetooth radio died or it lost pairing with the TV.
FIX First, re-pair it. Unplug the TV for 30 seconds. Plug it back in. Immediately point the remote at the TV and hold the pairing button (in the battery compartment) for 5 seconds. Wait for the pairing screen.
IF THAT DIDN'T WORK The remote is likely dead. Use the Roku mobile app as a temporary remote. Order an official Roku Voice Remote replacement. Third-party remotes often have pairing issues.
HEADS UP Check the battery contacts inside the remote. They can get compressed and not make contact. Bend them up slightly with a small tool.