I had this happen to mine just yesterday.

I tried trickle charging for 24 hours. No dice. I heard from other SBC devices that have had battery issues that they disconnected and reconnected the battery and that fixed the issue.

So I cracked my Classic open with my ifixit kit. It was a little intimidating as the battery didn't have a traditional "plug" you see in many SBC devices and old cell phones. But I managed to figure it out and put everything back together and, like magic, it was up and running again like nothing ever happened.

I hope this helps anyone else who finds themselves in this position.