Sorry to hear that.
There's a few reports of that message for Kindle devices. You probably saw them.
Here is one that recommends freezing the device : (
https://www.mobileread.com/forums/sh...d.php?t=326181) which didn't end in success.
(I've frozen dead laptop batteries in the past)
Is it a battery issue? I have no way of knowing - so proceed with caution. But if replacement battery isn't expensive I'd order one now in any case.
I wonder if someone like Booxtor with experience with Onyx repairs could help.
By the way, where are you? I cracked the screen of my M96, which was otherwise fine, and am looking to get rid of it (but I'm in China).
How old is your device?
Best of luck.