The two failures that look the same and are not
A Roku simple remote is infrared. It has no pairing, no wireless link and nothing to lose, so it fails for exactly two reasons: the batteries are flat, or something is standing between it and the device. Roku is explicit that these remotes have no pairing button, which means every "hold the pairing button" instruction you find online simply does not apply to yours.
A voice remote is a different animal. Some models talk to the player over wireless signals rather than infrared, which means they hold a relationship with a specific device — and relationships can break. A firmware update, a power cut or a long spell without batteries can leave it searching for a player it can no longer see. That failure needs re-pairing, and no amount of new batteries will fix it on its own.