Have you powered off the TV and Sonos, by unplugging them from the wall for two minutes? Have you called Sonos Support directly to discuss it?

I wouldn’t change the LG remote’s setup, those instructions are designed for connection with IR, which is the Sonos PLAYBAR, PLAYBASE, and Ray. With your Arc, you want the commands to travel via CEC across the HDMI connection, from the TV to the Arc. Which also raises the question, is the Sonos Arc connected to the TV’s ARC output port via HDMI?

Presumably you’ve re-run Remote Control Setup in the settings for the ARC in the Sonos app?

@StrayCat17 

You can use both remote options (hdmi cec and infrared) parallel on Sonos Arc. You can configure an ir remote via Sonos app in Sonos Arcs settings and use that one also to adjust volume if the tv is turned off. On the other hand when tv is turned on volume control via hdmi cec should work if cec connection works properly. So maybe you have to renew cec connectivity… turn it off in tv setrings, turn off tv and Sonos Arc, remove hdmi cable at tvs hdmi eARC port, turn on both devices, turn on cec option (simplink) on tv and replug hdmi cable. 
 

If your cable box remote supports ir you can also try to configure it via Sonos app as a second option. 

That would only be used for ir and not bluetooth remotes, though?

Thanks for everyone for your responses. A few answers to those who asked questions:

• @Airgetlam The TV is mounted on the wall with a plug behind it, so I don’t think I can unplug it right now. I did unplug the Sonos Arc and replug, but that didn’t work.  And yes, it’s connected via the HDMI/e-ARC port on my TV.  As for calling Sonos support, that was the plan until I realized they were closed all weekend.  May try tomorrow.
• @Rhonny I did try running the Sonos app’s remote set up.  But if I click off “Use IR” or “IR Signal” then it doesn’t allow me to run “Remote Control Setup.”  If I leave them on and run Remote Control Setup, it asks if my remote is compatible, and then says it’s not compatible unless I follow their specific directions.  But their specific directions don’t match up with what I see on my TV, and even when I get close to doing what they tell me to do, it doesn’t work.
• @Schlumpf I will try running my cable remote through the Sonos app to see if that helps with that particular remote, but that would be weird if it worked and not the TV remote after it previously being the opposite.  Worth a shot though!  I did toggle on/off the CEC but I guess I have to completely reboot the whole system? Will try that as well!
• @106rallye The TV remote is RF, not IR.  Sonos says I can use an RF remote if I set certain options on my TV, but I have looked and looked and those options don’t show up anywerhere.  And no bluetooth remote, that’s correct.

Thanks again everyone!  Will report back if something here works.

If the CEC component in the TV is ‘borked’, perhaps by a power fluctuation, you will end up needing to power cycle the TV, as inconvenient as that might be. It’s the only way to force a reload of the firmware / OS that runs the TV set. That would be the same issue that @106rallye is commenting on, the RF would contact the TV set, and send a CEC compliant signal through to the Arc. 

@StrayCat17 

I also think you should find a way to replug your tv. Otherwise most options to get it sorted won’t work at all. 
Also the cec renewing I described will need that. 

@StrayCat17 

BTW...make sure nothing is blocking the remote sensor on the Arc (click here). If all OK...then try the following suggestion….

If you remove the Arc from your TV does either remote operate the TV volume? I’d mainly be concerned with the TV remote as it is the lynch-pin. If doesn’t I’d call LG for asssitance to reset it.

If the remote does operate the TV volume I’d run the setup for the Arc again 

You can power down the TV using a house fuse or circuit breaker. Before doing this, check for anything else on that circuit that might get upset. I recommend that the HDMI cable be detached as ARC and TV reboot.