I have had to unplug the Roku (I have a Roku Ultra) AND take out the batteries to fix the problem. Plugging it back in and reinstalling the batteries fixed it - for now. If you have a Roku TV or a stick, I have no idea about those.
If this doesn't work, try throwing the remote against the wall a few times. It probably won't correct the issue but it does help relieve a little tension.
I have this same problem, but my remote is the new rechargeable remote, so I can't take out the batteries 😢