Is it showing up on your router info? - showing as connected to wifi? I only ask as I had a similar issue this morning with one of my Era 300s (one of a stereo pair). It was showing up on my router as connected and could even play the music already queued to them both. Just would not show on my app. After rebooting it didn’t work, I then rebooted another sonos speaker and low and behold - it showed up again and was fine. I have no idea why this happened.

If it’s completely locked up and you cannot power it off by long pressing the rear (top) power button for 5 seconds, or longer, (whilst it’s off it’s charger), then first maybe either wait to see if it powers off after 30+ minutes of inactivity (again leave it off the charger and do not open the Sonos App) or perhaps consider removing it’s battery for 30 seconds as outlined in this Sonos support link:

https://support.sonos.com/en-us/article/replace-the-battery-in-your-sonos-move

It can be done quite easily with a hex screwdriver, or Allen key and it does not invalidate any existing warranty if you follow the guide.

A further alternative is to wait until the battery level becomes so low that the device powers off, but that may take quite some time.

If problems still persist, then it maybe best to submit a Sonos system diagnostic report from within the Sonos App and either note/post it’s reference back here and then contact/chat with Sonos Support Staff via this LINK  and see what the Staff can perhaps suggest to resolve the matter.

try this